[P2P] Integrate background router (#732)

## @Reviewer
This PR may be more digestible / reviewable on a commit-by-commit basis.
Commits are organized logically and any given line is only modified in a
single commit, with few exceptions*.

*(In the interest of preserving the git-time-continuum
:police_officer::rotating_light:, this applies in batches of commits
between comments or reviews *by humans*, only once "in review")

---

## Description

### Before

While `backgroundRouter` exists and implements the `Router` interface,
it had yet to be connected to anything. Additionally, it was not able to
handle incoming unicast messages.

```mermaid
classDiagram
    class p2pModule {
        -router Router
        -handlePocketEnvelope([]byte) error
    }

    class P2PModule {
        <<interface>>
        GetAddress() (Address, error)
        HandleEvent(*anypb.Any) error
        Send([]byte, Address) error
        Broadcast([]byte) error
    }
    p2pModule --|> P2PModule

    class RainTreeRouter {
        UnicastRouter
        -handler MessageHandler
        +Broadcast([]byte) error
        -handleRainTreeMsg([]byte) error
    }

    class BackgroundRouter {
        UnicastRouter
        -handler MessageHandler
        +Broadcast([]byte) error
        -handleBackgroundMsg([]byte) error
        -readSubscription(subscription *pubsub.Subscription)
    }

    class UnicastRouter {
        -messageHandler MessageHandler
        -peerHandler PeerHandler
        +Send([]byte, Address) error
        -handleStream(libp2pNetwork.Stream)
        -readStream(libp2pNetwork.Stream)
    }
    RainTreeRouter --* UnicastRouter : (embedded)

    p2pModule --o "1" Router
    p2pModule ..* RainTreeRouter : (`router`)
    
    class Router {
        <<interface>>
        +Send([]byte, Address) error
        +Broadcast([]byte) error
    }
    BackgroundRouter --|> Router
    RainTreeRouter --|> Router
```

### After

`backgroundRouter` embeds `UnicastRouter` and is integrated into
`p2pModule` such that it is considered when calling `P2PModule#Send()`
and `P2PModule#Broadcast()`.

```mermaid
classDiagram
    class p2pModule {
        -stakedActorRouter Router
        -unstakedActorRouter Router
        -handlePocketEnvelope([]byte) error
    }

    class P2PModule {
        <<interface>>
        GetAddress() (Address, error)
        HandleEvent(*anypb.Any) error
        Send([]byte, Address) error
        Broadcast([]byte) error
    }
    p2pModule --|> P2PModule

    class RainTreeRouter {
        UnicastRouter
        -handler MessageHandler
        +Broadcast([]byte) error
        -handleRainTreeMsg([]byte) error
    }

    class BackgroundRouter {
        UnicastRouter
        -handler MessageHandler
        +Broadcast([]byte) error
        -handleBackgroundMsg([]byte) error
        -readSubscription(subscription *pubsub.Subscription)
    }

    class UnicastRouter {
        -messageHandler MessageHandler
        -peerHandler PeerHandler
        +Send([]byte, Address) error
        -handleStream(libp2pNetwork.Stream)
        -readStream(libp2pNetwork.Stream)
    }
    RainTreeRouter --* UnicastRouter : (embedded)
    BackgroundRouter --* UnicastRouter : (embedded)

    p2pModule --o "2" Router
    p2pModule ..* RainTreeRouter : (`stakedActorRouter`)
    p2pModule ..* BackgroundRouter : (`unstakedActorRouter`)
    
    class Router {
        <<interface>>
        +Send([]byte, Address) error
        +Broadcast([]byte) error
    }
    BackgroundRouter --|> Router
    RainTreeRouter --|> Router
```

## Issue

- #505 

## Dependencies

- #844 

## Type of change

Please mark the relevant option(s):

- [x] New feature, functionality or library
- [ ] Bug fix
- [ ] Code health or cleanup
- [ ] Major breaking change
- [ ] Documentation
- [ ] Other <!-- add details here if it a different type of change -->

## List of changes

* added background message protobuf type
* added `Router#Close()` interface method
* separated raintree & bg protocol IDs
* move `PocketEnvelope` nonce generation to `PackMessage()`
* added `Handler` to router configs
* refactored raintree router to support P2P module & `Router` interface
changes
* refactored raintree router bootstrapping
* refactored background router to support P2P module integration &
`Router` interface changes
* integrated background router into P2P module
* refactored staked actor router (raintree) peer discovery
* updated tests, post-refactoring


## Testing

- [x] `make develop_test`; if any code changes were made
- [ ] [Docker Compose
LocalNet](https://github.com/pokt-network/pocket/blob/main/docs/development/README.md);
if any major functionality was changed or introduced
- [x] [k8s
LocalNet](https://github.com/pokt-network/pocket/blob/main/build/localnet/README.md);
if any infrastructure or configuration changes were made

<!-- REMOVE this comment block after following the instructions
 If you added additional tests or infrastructure, describe it here.
 Bonus points for images and videos or gifs.
-->

## Required Checklist

- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have added, or updated, [`godoc` format
comments](https://go.dev/blog/godoc) on touched members (see:
[tip.golang.org/doc/comment](https://tip.golang.org/doc/comment))
- [x] I have tested my changes using the available tooling
- [ ] I have updated the corresponding CHANGELOG

### If Applicable Checklist

- [x] I have updated the corresponding README(s); local and/or global
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have added, or updated,
[mermaid.js](https://mermaid-js.github.io) diagrams in the corresponding
README(s)
- [ ] I have added, or updated, documentation and
[mermaid.js](https://mermaid-js.github.io) diagrams in `shared/docs/*`
if I updated `shared/*`README(s)

---------

Co-authored-by: @Olshansk <[email protected]>
Co-authored-by: Daniel Olshansky <[email protected]>
Co-authored-by: d7t <[email protected]>
Co-authored-by: Daniel Olshansky <[email protected]>

Makefile

@@ -301,7 +301,7 @@ protogen_local: go_protoc-go-inject-tag ## Generate go structures for all of the
 	$(PROTOC_SHARED) -I=./consensus/types/proto --go_out=./consensus/types ./consensus/types/proto/*.proto
 
 	# P2P
-	$(PROTOC_SHARED) -I=./p2p/raintree/types/proto --go_out=./p2p/types ./p2p/raintree/types/proto/*.proto
+	$(PROTOC_SHARED) -I=./p2p/types/proto --go_out=./p2p/types ./p2p/types/proto/*.proto
 
 	# echo "View generated proto files by running: make protogen_show"