Add Redis server example

[trustin]

Motivation:

• We have a Redis client example, but not its server counterpart.
• This repository is not very usable at its current form because it depends on Netty 5.0.0.Final-SNAPSHOT which is actually very outdated.

Modifications:

• Revert the repository to use Netty 4.1.
• Add RedisServer and RedisServerHandler which demonstrate how to write a server that understands RESP.
  • Currently supports GET, SET, DEL and SHUTDOWN command in a very simplistic form.

Result:

• This repository now builds without failure.
• This repository now uses the Netty version that's actually up to date.
• We now have a Redis server example, as well as a client-side one.

[chrisvest]

For Netty 4.1, we have the redis code in our main netty/netty repository. This contrib repo is here so we could remove that module from the Netty 5 main branch.

[trustin]

@chrisvest Then shall I send a PR to netty/netty#4.1 then?

[chrisvest]

@trustin For adding an example server based on 4.1, I think that's the correct place.

[sh-cho]

@trustin I'd like to continue this on netty/netty, if you don't mind.
Although redis codec will be removed main repo, example of redis server implementation will work well until someday netty5 is released.