api: extract base class

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.

test/mod/test_monitor.py

@@ -93,7 +93,7 @@ class TestMonitor(unittest.TestCase):
                 p.start()
                 p.join()
                 self.assertEqual(p.exitcode, 0)
-                output = api.output
+                output = api.output  # pylint: disable=no-member
                 assert output
 
             self.assertEqual(json.dumps(args), output)