Improve RoundRobinGroup feature

When the router get a new connection and call Router::doConfig, it was
adding by agent, the configurations of the new agent in the filter pool.
It is fine most of the time, but when each agent is configured with all the
campaigns, it creates a big overhead having to filter this big pool each
time.

Instead, now, the filter pool is given the round robin group name and the
last configuration it received from the agent.

The router::unconfigure member function is used to release the
configuration of one agent and take care of removing the configuration from
the filter pool if needed.