Extract post-transaction hook for Task callback

[svevang]

The Task model can trigger async work before the wrapping transaction is committed. Sometimes this can cause dirty reads from the async worker's read-committed view of table state.

#788 was a workaround for this problem in the case of publishing lock state (and can be backed out of when this is solved).

Instead of using Rails model hooks and other callbacks to schedule work, look at ways to trigger the work after the transaction commits -- ensuring that each reader to database state is at the same PostgreSQL MVCC isolation level.

• Create something like a "task epilogue" that the task subclasses can leverage
• Examine each task to find where async work is fired off and move that

[svevang]

Maybe call it a Task#on_success hook