Return new ReflectData for both reflect datum reader and writer

[RustedBones]

addresses #28279

Avro has a discrepancy between reflect reader and writer side:

• In the reader side, new ReflectDatumReader<>(clazz) returns a new model data
• In the writer side new ReflectDatumWriter<>(clazz) returns the singleton model data

Updating the model data's logical types is not a thread safe operation, so we must not use the singleton instance in this case.

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[mosche]

Interesting, constructors are not even consistent within ReflectDatumReader. Just that single constructor creates a new ReflectData instance, all the other ones use the shared instance.

Thanks a lot for addressing this inconsistency, this makes absolute sense. LGTM 💯

Though, nevertheless, I do wonder why/how this manifests as NPE at this position looking at your stack trace.