[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.

[check-manifest]
ignore =
  debian
  debian/*
  cubicweb.spec
  __pkginfo__.py