max_slave_replication_lag should be in mysqlmon, not readwritesplit

The MySQL monitor should handle slave servers that are too much behind the master. By removing the Slave status from any server that lags too far behind, the same functionality is achieved that is currently in the readwritesplit.

This also allows for some rudimentary IO health checks by checking that the insert into the heartbeat table can be done. If it can't be done then the master is not fully functional.