0.6.0-beta.2 Nats persistent storage

We've got persistent storage! Using Nats..

New beta, new architecture it seems. This time it comes with designs (see bottom of the README). If you didn't implement your own scheduler, this version's only big change is the adding a reference of the scheduler when creating/removing notifications of a job. I like this architecture and it made implementing it on top of Nats very easy. I would expect porting it to other storage providers would be easier.

See migration.md for migrating from previous versions to this.

There's some PRs and issues I'd like to get into before doing 0.6.0 release proper. But this one is huge in code and refactoring wise.