New class RPC::Status enforces JSON-RPC 2.0 error format.

* Relevant issues:
  * RIPD-92
  * RIPD-97
  * RIPD-98
  * RIPD-439
Fix outgoing bytes calculation in HTTP server.
Call Stoppable::stopped in PeerFinder onStop.
Disable transaction submission tests under Travis.
Refactor SerializedTransaction:
* Use boost:tribool instead of two intertwined bool variables
* Trim down public interface, reduce member variables