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.
But since the queue is there anyway, the overhead should be negligible, I guess (except if constructing/copying a `Variant` is substantially expensive). Provided that's the case, I'd prefer the most easy to understand code.