big rework of the session-parser-interaction;

dynamically adding pool methods/properties as parser commands;
dynamically executing selected pool method/property;
greatly simplified `ControlSession` class;
removed the need for hard-coded command names;
adjusted unittests accordingly

src/asyncio_taskpool/exceptions.py

@@ -63,13 +63,5 @@ class ServerException(Exception):
     pass
 
 
-class UnknownTaskPoolClass(ServerException):
-    pass
-
-
-class NotATaskPool(ServerException):
-    pass
-
-
 class HelpRequested(ServerException):
     pass