The default address will be set from the environment, or fall back to
trying 127.0.0.1:9051. There is now an API to allow this to be
overriden as well.
Improved parts of the conversation implementation, moved the
conversation event monitor to Identity, added event monitor
population, and other minor changes.
Also, slightly refactor Ricochet to be an interface to get the various
top-level components, and let the implementation define it (such as
backend's RicochetCore RPC type)