[server] implement dynamic database pooler Opening too much database connection has a cost at startup and also PostgreSQL as a maximum number of connection (100 by default). This get worse when starting multiple wsgi processes, since each process has its own database pool. Instead of opening `connections-pool-size` connections to the database at startup, just open one and open more only when needed.

[server] use a LifoQueue in _CnxSetPool In postgresql, some cache is attached to the connection. Using a LifoQueue (last-in, first-out) makes a few connections to get the most load which give best performance.

[server] extract creating a new cnxset in a _new_cnxset() helper So we can move logic specific to _CnxSetPool here.

[server] avoid a possible race condition on _CnxSetPool.close() The pool could become empty between time to check and time to use.