Changed RedisSubscriber wait with TaskCondition vs Timers

This update provides stability and performance improvements to the `RedisSubscriber`:

- Uses `TaskCondition` notifications rather than rearming timers throughout the class.

- Permits queuing actions in case multiple tasks are communicating, the old Timer/null check method had undefined behavior.

- Fixes an error with the `ConnectionPool`, where connections could be re-used after they were closed. An assertion on the new event driver required this fix.
Okay, I see. Maybe it makes sense to convert the nested functions into private member functions instead, so that the function gets a bit more manageable. But that doesn't need to be done for this PR.