OpObservers fire whenever oplog entries are created and are about to be persisted, and represent a way for arbitrary server subsystems to receive notification of durable events.

We should create a new OpObserver for user write block mode.
This OpObserver should check whether user write block mode is active by checking UserWriteBlockModeState. If blocking is enabled, the observer should throw an exception if the oplog entry it's observing is a user write. This will prevent the oplog entry from being applied.

We should register the observer on standalones, replicasets, and shard servers. Config servers don't need this hook.