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.
OT: Maybe we should add a method `RedisClient.createSubscriber` to create the instance. That way we get at least some freedom in the future to change what kind of storage/RC/... is used.