[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.

.. _session_module:

:mod:`cubicweb.pyramid.session`
-------------------------------

.. automodule:: cubicweb.pyramid.session

    .. autofunction:: includeme

    .. autofunction:: CWSessionFactory