ReplicaSetMonitor should monitor arbiters

From SDAM:

The Client MUST monitor arbiters. Mongos 2.6 does not monitor arbiters, but it costs little to do so, and in the rare case that all data members are moved to new hosts in a short time, an arbiter may be the client's last hope to find the new replica set configuration.

http://emptysqua.re/server-discovery-and-monitoring.html#the-client-must-monitor-arbiters