Python Loadbalancing capabilities

Python connector should provide that some load balancing capabilities.

There can be different principles:

• selector like 'RR' (round-robin), 'RANDOM' or 'ORDER' multiple hosts of the same type (main or replica)
• have a counter to know the number of connection for each connection.

The first implementation is easier to do.
The second implementation has the advantage that when a node failover is ready again, to directly report the load to this new node until the distribution is similar on all nodes.