Generalize QueuedNotificationManager

QueuedNotificationManager is a useful utility for scheduling in-order execution of tasks. Unfortunately its design is heavily affected by the use case it comes from, which limit its usefulness:

1. listener keys work on identities, which prevents common equality to be used – not all potential users can work with identities reasonable (i.e. they have to do String.intern())
2. there is no provision to control timing of submission of individual entries, i.e. users cannot specify a timeout for single task submission
3. there is no provision to return tasks not published within a certain deadline

Address these issues by generalizing these use cases into a new class and make QueuedNotificationManager a special-case of that class.