Re-implement Mongo.unlock and Mongo.isLocked with new database commands

The Mongo.unlock method currently queries the "$cmd.sys.unlock" method to unlock the server. The 3.2 server will support an fsyncUnlock command to replace this. The driver should call that command if it detects a 3.2 server. Otherwise it should ball back to the existing way of doing it.

Similarly, the Mongo.isLocked method queries the "$cmd.sys.inprog" collection. The 3.2 server will support a currentOp command that does the same thing. The driver should call that command if it detects a 3.2 server. Otherwise it should ball back to the existing way of doing it.