Split out the part of `api.API` that is responsible for providing
the server infrastructure for the API; i.e. setting up the server
and the corresponding context manager and asynchronous event
handling. This leaves `API` itself which just the implementation
of the high level protocol and makes the API-server part re-usable.
NB: pylint, for some reason, confuses `API` and `BaseAPI`, like in
`test_monitor`. Annotate that accordingly.
1ce517e595