[k2] gossip module (protocol spec and implementation)

[neonphog]

Soft dependency on #26 (we can start work on this before 26 is done, because we know it will just fit into a byte array somewhere in there).

The gossip module is going to need it's own protocol to communicate with remote peers. Since we've landed on protobuf for #26 we probably want to use that here too.

The first iteration of the gossip protocol should be the minimum required to implement the syncing by time slice and arc rage as define in #27 . We don't have to have the protocol perfect and future-proof for upgrading yet : )

[neonphog]

Awaiting some more work on the core gossip before knowing better what this will look like.

[pdaoust]

@ThetaSinner

protobuf

was this a discussion had elsewhere? #26 says MsgPack. Curious whether this change to Protobuf will be invisible to the dev, or whether some Protobuf-serialised data will bubble up to the HDK or client.

[neonphog]

@pdaoust

Discussed in this PR. No this is just wire protocol, app devs will never deal with protobuf.