BackgroundSync and InitialSyncer both call OplogBuffer::waitForSpace, then OplogApplier::enqueue, which calls OplogBuffer::pushAllNonBlocking. This is a roundabout way of doing a blocking enqueue, and it requires BackgroundSync and InitialSyncer each to know about both the OplogApplier and its internal OplogBuffer.

We will make OplogApplier::enqueue blocking, and make its callee OplogBuffer::push blocking too. This simplifies BackgroundSync/InitialSyncer and decouples them from OplogBuffer.