Add support for ReplicaSet connection and WriteConcern

Add support for connection to Mongo DB replica sets.

Add support for Mongo DB write concern.

Note: There seem to be some kind of bug with rJava and the java mongo driver when directly connecting to a replica set: if the first operation is to connect to a replica, all subsequent operations hang. To avoid the issue, when connecting to a replica set, it first connects as if there wasn't a replica, close the connection and then reconnect to the replica.