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.
So, I shouldn't protect the TaskCondition with a TaskMutex? I was certain it had to be used like the pthread mutexes.

Also, would you suggest using a mutex in the RedisClient to protect `_request_void`?