Handle dual writes for remaining merge_data attributes

What does this MR do and why?

We've implemented dual-write for merge_status in !208723 (merged)

Now this MR covers dual-writes for remaining merge_data attributes.

This MR overrides attribute setters to handle direct assignments as well as mass assignment via new, create, update.

It also introduces wrapper methods for handing partial merge_params updates as well as direct column updates for merge_jid and merge_ref_sha.

merge_data will be saved once mege_request gets saved and I tried to keep the existing code as is to minimize disruption.

References

#560933

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Related to #560933