Issue #449: AvroConsumer: provide method to get schemas from last message decode

[soxofaan]

This is an alternative take on issue #449 that tries to address some concerns raised in PR #453 :

• overhead is minimal (no mixins, no dynamic wrapper type creation and other magic), so not being able to turn it off is less of a concern
• no non-optional _schema attribute on the wrapper that might cause collisions or raise code style complaints for accessing a "private" attribute

On the other hand, this solution is conceptually bit uglier than #453 as the schema (id) is not associated with the returned message directly and has to be requested with an explicit calls.

Very rough usage example:

consumer = AvroConsumer()

# Get a message
msg = consumer.poll()

# Get key/value schema ids and schema objects
key_sid, value_sid = consumer.last_decode_schema_ids
key_schema, value_schema = consumer.last_decode_schemas()

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[soxofaan]

Please note: this is proof of concept to collect initial feedback (e.g. from @saabeilin, @rnpridgeon, @edenhill, involved in #453)
I'm planning to finetune documentation a bit and add tests

[soxofaan]

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[soxofaan]

rebased branch to resolve merge conflicts

[rnpridgeon]

Generic Serde support will provide a means to bind your schema with your data. This is similar to java's 'GenericRecord' implementation. #502