Remove MongoFuture and moved to callback

JAVA-1554
Seems like we could DRY this up if the handler managed the latch, translated InterruptedException (but to a MongoInterruptedException), and threw the error.  So it would look something like:

        FutureAsyncCompletionHandler<Void> handler = new FutureAsyncCompletionHandler<Void>();
        writeAsync(buffers, handler);
        handler.get()