Tidy up flow-networking usage

[SpaceManiac]

Since the MC 1.8 update, Glowstone has been using a custom Netty pipeline different than the one provided by flow-networking. Some methods in GlowProtocol in particular are mandated by flow-networking for use in its pipeline but are not currently used. This should be cleaned up.

[SpaceManiac]

The flow-net components which Glowstone currently uses directly are:

• Message, an empty interface with some docs, and AsyncableMessage (boolean isAsync()).
• Codec (T decode(ByteBuf), ByteBuf encode(ByteBuf buf, T message)).
• MessageHandler (void handle(S session, T message)).
• ByteBufUtils.
• ConnectionManager (trivial to refactor out).
• Session and BasicSession.
• HandlerLookupService, CodecLookupService, CodecRegistration.
• AbstractProtocol in some respects (interface differences are hackily worked around).
• Exception classes (barely actually used).