Support AllIdleConnections and AllConnections for flush strategy

Add the flush strategy of 'AllIdleConnections' which will remove all idle connection from the entire pool, and 'AllConnections' which will remove all connections from the entire pool.

The existing 'IdleConnections' will now only remove idle connections in the active sub pool, and 'EntirePool' will now remove all connections in the active sub pool. The sub pool is based on the credential separation.