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.
Create a new monitor that records all the invocations of the
monitoring (virtual) functions and use that to check that when
running (i.e. building) a pipeline all of them are executed
the excepted number of times (and with the correct arguments).
Add a basic test that will set up an 'API' endpoint, then spawn a
child process that uses that 'API' endpoint to setup its stdio in
very much the same way as runners do. This is used to verify that
the API itself works properly as well as the new LogMonitor class
by comparing the inputs and outputs.
