fix: Enable reading from secondaries in the MongoDB driver.

This patch changes the MongoDB driver so that it will use
MongoReplicaSetClient instead of MongoClient when a
replicaSet is specified in the connection URI. Testing
revealed that this is necessary for readPreference to be
respected.

Change-Id: I3c1a2a166c5dd2c857cd52a2d1766437a9e4a940
Merge "fix: Enable reading from secondaries in the MongoDB driver."