Flatbuffers serialization for Raft Messages

[hkadayam]

Currently we use Protobuf serialization for Raft messages. Given that we never mutate the serialized objects and serialize them when we are sending the message and never mutate them on receiving side, flatbuffers schema could be more useful.

Note that the data channel headers are going to be flatbuffers serialized

This issue is track an effort to see if its useful to convert to flatbuffers (should be relatively straightforward)