NIFI-3682: Add "Schema Access Strategy" and "Schema Write Strategy" Record Readers and Writers; bug fixes.

[markap14]

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[joewitt]

We have avro schema as the basis for carrying the schema data for readers and writers but appear not to use the concept of defaults. We should take advantage of that for reading/writing. Same for aliases presumably. This also increases the opportunity to have a reading schema and a writing schema match up based on aliases.