Fix a bug in Replica Set monitoring in split brain situation

Suppose you have a Replica Set and a MongoReplicaSetClient connection to
it which you don't close.

1. At some time you get a split brain situation and a part of the
   Replica Set nodes become isolated from others.
   MongoReplicaSetClient.refresh() will remove them from its RSState and
   will monitor only the other part of nodes.
2. After some time your host loses connection to the second part of
   nodes. Replica Set monitor will continue to monitor only the second
   part of nodes.
3. And finally after some time connection to the first part of nodes
   resumes but to the second part not. But the Replica Set monitor
   continues to monitor only the second part of nodes and your
   connection won't work until at least one host from the second part
   become available.

This commit fixes the issue.