Implement hook for pausing / resuming the replicator

1. Create pause / resume hook that derives from BGHook.
2. Create methods on the replicator fixture for sending the PAUSE / RESUME
3. Call pause resume method from the hook. There will be some bookkeeping with whether the replicator is alive or not that we'll have to think about as well, because we can't call pause when the replicator is dead.
Likewise, we will also have to think about what happens when a paused replicator is killed, and then the pause / resume hook tries to resume it before it has been restarted.