Psycopg2-mq

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- Attempt to reconnect to the database after losing the connection.
If the reconnect attempt fails then crash.

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- [model migration] Add a ``worker`` column to the ``Job`` model to track what
worker is handling a job.

- Add an optional ``name`` argument to ``MQWorker`` to name the worker -
the value will be recorded in each job.

- Add a ``threads`` argument (default=``1``) to ``MQWorker`` to support
handling multiple jobs from the same worker instance instead of making a
worker per thread.

- Add ``capture_signals`` argument (default=``True``) to ``MQWorker`` which
will capture ``SIGTERM``, ``SIGINT`` and ``SIGUSR1``. The first two will
trigger graceful shutdown - they will make the process stop handling new
jobs while finishing active jobs. The latter will dump to ``stderr`` a
JSON dump of the current status of the worker.

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- Only save a cursor update if the job is completed successfully.

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- Mark lost jobs during timeouts instead of just when a worker starts in order
to catch them earlier.

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- When attempting to schedule a job with a cursor and a ``scheduled_time``
earlier than a pending job on the same cursor, the job will be updated to
run at the earlier time.

- When attempting to schedule a job with a cursor and a pending job already
exists on the same cursor, a ``conflict_resolver`` function may be
supplied to ``MQSource.call`` to update the job properties, merging the
arguments however the user wishes.

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- [model migration] Add a new column ``cursor_snapshot`` to the ``Job`` model which
will contain the value of the cursor when the job begins.