Improvements to MongoDB replica sets

* Replica set class is Iterable<MongoDBServer>
* Replica set entity sets state to on fire if first server up fails to
  initiate the set
* Logging in MongoClientSupport improved when command result was not ok.
Switch Iterable<MongoDBServer> for getReplicas() method
Merge pull request #1097 from sjcorbett/mongodb-tweaks

Small improvements to MongoDB replica sets