VReplication: Ensure that RowStreamer uses optimal index when possible

[mattlord]

Description

In this PR we add a force index clause (and not always primary) when we have PK or equivalent columns covered by an existing index, and we then use all of those columns in the ORDER BY clause. So it’s for the low level rowstreamer source query we apply at the mysqld layer to get rows for streaming, before applying the vreplication filters in the vstreamer layer in the source tablet and finally sending the matching rows on to the vcopier in the target tablet. Using an index for the ORDER BY clause there in the mysqld layer is what allows us to immediately start streaming rows rather than having to read every matching row and sort them all on disk with a filesort in mysqld before the vstreamer gets any rows back from mysqld/rowstreamer.

Note
✅ My current plan is to hold off on merging this until after the upcoming 18.0-RC cutoff (scheduled for Sept 29th) so that it has the full v19 dev cycle to bake and surface any unforeseen edge cases.

Related Issue(s)

• Fixes: vreplication: Materialize stalls when unique field left out #11991
• Fixes: Bug Report: VReplication copy phase slow/stuck due to not using optimal index for resume query #13919

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

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[rohit-nayak-ps]

lgtm

[shlomi-noach]

Makes perfect sense, with one minor comment.