Upgrade to node-mongodb-native 1.4.1

This improves the replset join event.
Re-poll all OplogObserveDrivers on mongo failover
Add a flag to MongoRunner which runs 3 mongods

This is only intended for testing. No data is preserved from one run to
the next, and if any mongod exits for any reason, the rest are killed;
there is no mongod restarting.

It takes a while (~20 seconds) to start up because it waits for the
replset to be ready.
Test that observeChanges works over a failover
Feedback from nim's review of be4d306.
History update for Mongo improvements