[server] extract "no pooler" CnxSet class to a _BaseCnxSet class So we get rid of "if self._queue is None" in each method of _CnxSetPool Also add helper get_cnxset(source, size) to instantiate the correct pooler class.

option_removed('host')
option_removed('uid')
option_removed('webserver-threadpool-size')
drop_entity_type('CWCache')