Make Raft messages serializable

- Make AppendEntries a protocol buffer message
- Create the Payload extension mechanism to be used for AppendEntries by extensions
  of RaftActors to define their custom Payloads
- Modified tests etc as appropriate

Change-Id: I2d5af41b7ca8ad7a8b94515ebc5adaa78803ad15
Signed-off-by: Kamal Rameshan <kramesha@cisco.com>
Signed-off-by: Moiz Raja <moraja@cisco.com>
Merge "Make Raft messages serializable"