AVRO-3048: Use SpecificRecord's MODEL$ in builder creation

[unchuckable]

This PR addresses AVRO-3048 (https://issues.apache.org/jira/projects/AVRO/issues/AVRO-3048), improving newBuilder() performance for newly generated SpecificRecords by reusing the already existing SpecificData instance (MODEL$) in the specific records themselves.

No new unit tests should be required, for the current changes are already covered by existing unit tests. Reference files for code generation have been updated to reflect the minor changes to the record and errror templates.

Further measures might be required to alleviate the performance regression of AVRO-3048 for existing code that cannot be re-generated easily.

If this PR is found feasible, I would suggest cherry-picking it for 1.9 and 1.10 branches, too, to allow for existing applications to benefit from the changes.

[RyanSkraba]

This LGTM! I think fixing the case for new generated records is the most important.

[RyanSkraba]

Even if this looks fine for addressing the original issue specifically raised in the JIRA, I asked another question there based on one of your comments. Can you take a look before we merge this?

A quick question though, I can't see how your PR reduces any calls to newInstance(...) (which is also an expensive call). Am I mistaken?

[unchuckable]

Even if this looks fine for addressing the original issue specifically raised in the JIRA, I asked another question there based on one of your comments. Can you take a look before we merge this?

A quick question though, I can't see how your PR reduces any calls to newInstance(...) (which is also an expensive call). Am I mistaken?

Good point, addressed this on JIRA.