This follows the README given earlier, at least in principle, but
doesn't implement the entirety of QMP, by any stretch of the
imagination.
Notable by their absence are error responses, argument validation
(for incoming and outgoing messsages of all types), and any visibility
into qmp_capabilities.
Also missing are integration tests.
The socket connector is responsible for reading from a socket and
converting the lines that were read to messages, using a passed-in
deserialiser. It also provides an interface to push messages onto
the socket, via a serialiser which is also passed in.
