Xbus.broker

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- Better message state tracking.

- Undelivered messages are re-sent when the recipient comes back.

- More logging.

- Documentation improvements.

- Upgrade step: Migrate the DB to version 1 using
<https://bitbucket.org/xcg/migrate_xbus_broker/>.

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- Update requirements.

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- Update the setup script wrt permission changes.

- Define required package versions in setup.py and document why some are
frozen.

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- Improve the README and configuration file paths.

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- Initial implementation of the Xbus broker in Python 3.