Notification::waitFor is not interruptible in 3.4

The Notification::waitFor call is not interruptible in the 3.4 branch and this could potentially cause deadlock at config server stepdown followed immediately by step-up, due to its usage in the sharding Balancer code. While the chance for such a deadlock is extremely rare, making Notification::waitFor interruptible should be very safe to include in 3.4.

We can just pick-up the changes to notification.h from this change.