Add basic version of asynchronous messages module (works with at most one synchro-peer for each peer).

Change-Id: I64152aa737b380347c0360943202caf8eb89aa6b
Use MessageSendFuture as Observable and ErrorResponder as its Observer.

Change-Id: I0aa65a38540092432b6ef3cd28caf99776c349d2
* Fix style issues
* Remove additional queues in Peer class

Change-Id: If6621eeece2e1385405b40bc5c84155f00048f7f