opt: reduce reptitive partial index and constraint predicate building

[mgartner]

We may build partial index and unique constraint predicates multiple times in optbuilder. For example, an INSERT ON CONFLICT will build a partial unique constraint predicate when determining arbiters and when building uniqueness checks. We should consider caching the built expressions to the table metadata.

Also, we may be able to eliminate any rebuilding of partial index predicates in arbiter-related code by using the predicates already built in the fetchScope (assuming we build and assign mutationBuilder.fetchScope before we try to find arbiters.

Finally, we build a partial unique constraint predicate thrice when building uniqueness checks for UPDATEs: to determine the column ordinals referenced in the predicate, to filter the WithScan, and to filter the Scan.

Jira issue: CRDB-3096

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