R1:84ee4cd9f62f

Factor out task execution and formalize permanent failures

Summary:
- Clean up a TODO about permanent failures.
- Clean up a TODO about failing tasks after too many retries.
- Clean up a TODO about testing for bad leases.
- Make the lease/retry implementation more flexible and natural.
- Make completely bogus tasks fail permanently.
- Make PhabricatorMetaMTAWorker use new `getWaitBeforeRetry()` (as intended), not hackily implement logic in `getRequiredLeaseTime()`.
- Document worker hooks…