Remove run_jobs_in_serial.

We should perhaps deprecate it instead of outright
removing it but a github search reveals no public
or internal moz projects are using it and given how
bad the perf is it’s best just to remove it.

Fixes #68.
Remove unused locals.
Store a cancellation time.
Require and persist a reason for a job batch cancellation.
Require and persist a creation_reason when a batch is created.