insights: Track Forwards using HtlcEventStream

[carlaKC]

The newly added HtlcEventStream added to lnd's routerrpc provides new insight into a node's forwarding activity. This data is useful for a variety of applications - rate limiting for forwards, success rate tracking of various channels, rebalancing hints which trigger when we see a drop in incoming traffic or a rise in ErrInsufficientBandwidth failures.

This issue proposes the addition of a rounting package that consumes forwarding events, and matches them to their settle/fails where applicable. Such a solution could also write htlcs to disk, but for now we will start in memory and let requirements take form. A rough version of this change has been started here.

Steps to Completion

1. Update lndclient to consume forwarding events (in loop repo)
2. Consume htlc events, classifying them into forwards, receives and link failures (where forwards contain a forwarding event and subsequent settle/fail events.
3. Add the ability to make requests to this subsystem for a set of events over a period of time, which are handled by the main event loop consuming htlc events from lnd so we don't need any locking.