Middleware Proof of Concept

[Fishwaldo]

Proof of Concept for Middleware

Just a simple Proof of Concept for Middleware...

A few things:

1. State handling needs to be stored just in the Schedule. I see some states in Schedule, and some in jobs (and for example, FINISHED state in Jobs, but the doco says a Finished State can not be rescheduled...)
2. the Schedule.transitionState is very simple now. It should handle:

• Selectively Blocking on some State Transitions
• Handle Panics (new State?)
• Handle rescheduling Jobs (eg, so you could implement retry/backoff via middleware)
• Handle Contexts when introduced?

[Fishwaldo]

btw, this is based Upon the Schedule Branch I had in the other PR.