Sprockets.mixins.avro-publisher

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- Add support for fastavro<2

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- Updated service to Python 3.7
- Switch from using gen.coroutine/yield to async def/await
- Update to Tornado 6
- Update all libraries to current versions. This may require switching to alternative libraries.

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- Consolidate code
- Streamline and use sprockets.mixins.http as well
- Replace tests with integration tests modeled after sprockets.mixins.amqp

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- Move Mixin to separate file
- Replace code with latest internal version
- Rename AvroPublishingMixin to PublishingMixin
- Update setup.py and requires files to current standard
- Replace avro library with fastavro library
- Add avro_amqp_publish helper method
- Add retry when schema cannot be fetched
- Delay before retrying is configurable via application.settings:
- avro_schema_fetch_retry_delay (default 0.5 seconds)
- Separate HTTP client from common app-based pool to help avoid excessive locking on high load
- Add unit tests
- Test execution requires a running AMQP server, see tests.py

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- Initial implementation

.. _Next Release: https://github.com/sprockets/sprockets.mixins.avro-publisher/compare/3.0.1...HEAD
.. _3.0.1: https://github.com/sprockets/sprockets.mixins.avro-publisher/compare/3.0.0...3.0.1
.. _3.0.0: https://github.com/sprockets/sprockets.mixins.avro-publisher/compare/2.1.0...3.0.0
.. _2.1.0: https://github.com/sprockets/sprockets.mixins.avro-publisher/compare/2.0.0...2.1.0
.. _2.0.0: https://github.com/sprockets/sprockets.mixins.avro-publisher/compare/1.0.1...2.0.0
.. _1.0.1: https://github.com/sprockets/sprockets.mixins.avro-publisher/compare/1.0.0...1.0.1
.. _1.0.0: https://github.com/sprockets/sprockets.mixins.avro-publisher/compare/7324bea...1.0.0