Psycopg2-mq

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- [model migration] Drop the ``UNIQUE`` constraint on the background job
``lock_id`` column.

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- [model migration] Add a scheduler model with support for emitting periodic
jobs based on RRULE syntax.
See https://github.com/mmerickel/psycopg2_mq/pull/11

- Enable the workers to coordinate on a per-queue basis who is in control
of scheduling jobs.
See https://github.com/mmerickel/psycopg2_mq/pull/12

- Reduce the number of advisory locks held from one per job to one per worker.
See https://github.com/mmerickel/psycopg2_mq/pull/12

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- Use column objects in the insert statement to support ORM-level synonyms,
enabling the schema to have columns with different names.

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- Ensure the advisory locks are released when a job completes.

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- Ensure maintenance (finding lost jobs) always runs at set intervals defined
by the ``timeout`` parameter.

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- Recover active jobs when the connection is lost by re-locking them
and ensuring they are marked running.