Support for triggering in-thread execution of `PeriodicTask`

[fmoo]

Maybe called trigger()?

[fmoo]

This method should return a Future object that can be blocked on until execution completes.

[fmoo]

Fixed by 99581d8

[fmoo]

My mistake, 99581d8 only implements the future portion of it; it doesn't trigger async execution. The remaining part should be easy to implement.