[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.

.. _req_module:

:mod:`cubicweb.req`
===================

.. automodule:: cubicweb.req

    .. autoclass:: RequestSessionBase
        :members: