schemadiff: improved heuristic for dependent migration permutation evaluation time

[shlomi-noach]

Description

See story in #14248. In this PR:

• We re-apply an ordering heuristic (where DROPs come first, then ALTERs, then CREATEs) before evaluating dependency permutations. This begins the search with a heuristically probable solution, and followed by "nearby" solutions.
• We copy less objects in the duration of the search. When evaluating a permutation we apply schema changes in-line. It's only at the beginning of a permutation search that we do one copy of the schema.
• We support context.Context for premature exit. This changes the function signature for OrderedDiffs(...)

Related Issue(s)

Fixes #14248

Checklist

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