This will allow registering subscribers with specific Managers. Since libmongoc dispatches events based on clients, it will still be possible for a Manager's subscriber to receive events from another Manager if both share the same client. That caveat should be documented, but it's unlikely to affect most users.

While not originally discussed in the scope for PHPC-1666, this implementation will additionally ensure that the same subscriber is not notified multiple times. That could otherwise happen if a subscriber was registered with two Managers sharing the same client, or with a single Manager and also globally.