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 for failures and retries.
• Provide coverage on everything.

Test Plan: Ran unit tests. Ran bin/phd debug taskmaster.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D3859