Limit total number of connections to backend

When the MaxScale pooling feature is enabled, idle connections are put to a pool from which connections are taken when needed. However, if there are no connections in the pool, then a new backend connection will always created. Consequently, the presence of the pool does not provide an upper limit for the total number of backend connections.

There are situations where providing an upper limit on the number of backend connections, while simultaneously not restricting the number of clients, is what is wanted. The pooling mechanism should be extended so that it also optionally would imply the maximum number of connections that are created.