Add support for replication sources

Allowing the monitors to only monitor a specific replication source would allow proper use of multi-source replication with MaxScale. Right now it is not possible to define overlapping clusters due to the fact that SHOW ALL SLAVES STATUS is used instead of SHOW SLAVE STATUS for a specific source. SHOW ALL SLAVES STATUS will tell if all source are replicated but it is not always the desired outcome.

Setting default_master_connection at the start of the monitor's session would allow the monitor to only monitor one replication source. This would allow multiple monitors to monitor the same set of servers without actually monitoring the same replication stream.