Wait to get a connection from exhausted pool
For now, on postgresql backend we use a connection pool with a default limit to 64. But this limit the number of thread the server can answer to also 64 because almost all requests request at least one connection. We can artificially increase this limitation by waiting for an available connection in the postgresql backend. Of course this is not a magical scaling solution and trytond (and postgresql) should still be correctly configured accordingly to the use case. But this solution allow to better resist in case of temporary over usage.