storage: update rule solver candidates to have 2 scores and a valid bool

[BramGruneir]

Do not merge until after #11695.
Ignore the first commit, it is part of #11695.

Two commits:
storage: convert rule solver rules to return 2 scores
This commit splits the scores returned in the rule solver into a constraint and a balance score.

storage: add a valid field to rule solver's candidates
Solve now returns all candidates instead of just the valid ones. To
get only the valid candidates, the new function onlyValid and new type
alias condidateList have also been added.

This change allows us to use solve for removeTarget. It also cleans
up the logic in removeTarget to more closely match the non-rule solver
version.

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This change is 

[petermattis]

Not finished reviewing this yet, just sending out some initial comments.

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Review status: 0 of 3 files reviewed at latest revision, 3 unresolved discussions, all commit checks successful.

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pkg/storage/allocator.go, line 310 at r3 (raw file):

		)

		if len(candidates) == 0 {

if len(candidates) != 0 {
  badStoreID = candidates[len(candidates)-1].store.StoreID
}

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pkg/storage/allocator.go, line 322 at r3 (raw file):

			return roachpb.ReplicaDescriptor{}, errors.New("could not select an appropriate replica to be removed")
		}
		badStoreID = bad.StoreID

if bad != nil {
  badStoreID = bad.StoreID
}

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pkg/storage/allocator.go, line 325 at r3 (raw file):

	}

	for _, exist := range existing {

if badStoreID != 0 {
...
}

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Comments from Reviewable

[BramGruneir]

Cool.

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Review status: 0 of 3 files reviewed at latest revision, 3 unresolved discussions, all commit checks successful.

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pkg/storage/allocator.go, line 310 at r3 (raw file):

Previously, petermattis (Peter Mattis) wrote…

if len(candidates) != 0 {
  badStoreID = candidates[len(candidates)-1].store.StoreID
}

Done.

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pkg/storage/allocator.go, line 322 at r3 (raw file):

Previously, petermattis (Peter Mattis) wrote…

if bad != nil {
  badStoreID = bad.StoreID
}

Done.

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pkg/storage/allocator.go, line 325 at r3 (raw file):

Previously, petermattis (Peter Mattis) wrote…

if badStoreID != 0 {
...
}

done, that's much cleaner.

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Comments from Reviewable

[petermattis]

Review status: 0 of 3 files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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pkg/storage/allocator.go, line 407 at r4 (raw file):

		existingCandidates := removeRuleSolver.Solve(existingStoreList, constraints, nil, nil)
		candidates := allocateRuleSolver.Solve(candidateStoreList, constraints, nil, nil)

Don't you need candidates = candidates.onlyValid(). This is exactly why I think there should be separate calls for removeCandidates and allocateCandidates rather than trying to shoehorn both into the same Solve interface. Even if under the hood {remove,alloc}Candidates use Solve, I think it is worthwhile to introduce those methods.

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Comments from Reviewable

[BramGruneir]

replaced by #12165