Queue-messaging

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- Backward incompatible update for latest google-cloud-pubsub compatiblity.
- PubSub message pulling becomes asynchronous
- messaging.receive instead of returning an Envelope with message now takes a callback to be called on Envelope
- google-cloud-pubsub will manage sleeping when pulling for new messages on its own
- Google Cloud project id is now required for pubsub configuration

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- Set not provided and not required fields to None.
- Removed pytest from dependencies.
- Use HTTP instead of gRPC by default, allow to pick gRPC instead in configuration.

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- First release.
- Simple high-level, provider independent API for receiving and sending structured messages.
- Validation when serializing data - protection against sending invalid data.
- Retry logic in case of connection error.
- Dead letter queue support. Send messages to another queue in case of an error.
- Custom MACAddressField for schema.