[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.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)
embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js