Define an execution model

[mehcode]

Currently jobs are run manually (with a helper method to run all currently pending jobs). We should define an execution model.

1. Does it make sense to have more than one execution model (sync vs async)?

2. How does the API look?

my_agenda.start()

3. How does the agenda, itself, execute (in a thread or coordinated with an event loop)?

4. How does each job execute (threadpool) ?

• https://github.com/frewsxcv/rust-threadpool
• http://alexcrichton.com/futures-rs/futures_cpupool/

[Boscop]

Are Jobs re-entrant?