[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.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)