Add more unit tests to rpc/impl/TransactionSign (RIPD-480):

By adding a mock it is possible to test the transactionSign
function without interacting with the ledger.  This is the
smallest change I could come up with that allows transactionSign
to be unit tested.

The unit tests are white boxed.  Each test case is a result
of examining the code and identifying behavior associated with
different JSON fields.  That means the tests are not based on
requirements, they are based on observed behavior.
Move the call to cancel_timer to the right place
Fixes to beast::asio::streambuf:
* Fix to_string conversion
* Fix assert on debug invariant checks
* Fix the treatment of the output position when the entire output is committed.
* Add unit test
Use injections from OverlayImpl in PeerImp
Use cluster state in Slot instead of PeerImp
Refactor protocol message parsing:

This replaces the stateful class parser with a stateless free function.
The protocol buffer message is parsed using a ZeroCopyInputStream.

* Invoke method is now a free function.
* Protocol handler doesn't need to derive from an abstract interface
* Only up to one message is processed at a time by the invoker.
* Remove error_code return from the handler's message processing functions.
* Add ZeroCopyInputStream implementation that wraps a BufferSequence.
* Free function parses up to one protocol message and calls the handler.
* Message type and size can be calculated from an iterator
  range or a buffer sequence.