[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.

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Multiple sources of data

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Data sources include SQL, LDAP, RQL, mercurial and subversion.

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.. XXX feed me