kvserver: update AdminRelocateRange to use explicit swaps of voters and non-voters

[aayushshah15]

AdminRelocateRange currently doesn’t make use of the ability to promote non-voters to voters, demote voters to non-voters or swap voters and non-voters. Because of this limitation, the mergeQueue has to resort to a fairly complex way of determining which targets to relocate a range’s replicas to during a merge in order to avoid redundant data movement.

There are a few problems with this:

• With the ability to swap replica types, the complexity mentioned above is unnecessary. We can avoid redundant data movement by performing all the promotions / demotions / swaps and then relocating the rest of the misaligned replicas.
• The merge queue relocation code was written before config: introduce num_voters and voter_constraints #57184. We’ve since introduced the concept of voter_constraints which govern the constraints over just the set of voting replicas. This means that the relocation algorithm computes results that may not necessarily meet the {voter_}constraints placed over the RHS. In such cases, the range merge will fail.
• It also has this limitation:

  cockroach/pkg/kv/kvserver/replica_command.go

  Lines 2547 to 2559 in df1c620

  	if len(finalRightVoters.Descriptors()) == 0 {
  	// TODO(aayush): We can end up in this case for scenarios like the
  	// following (the digits represent StoreIDs):
  	//
  	// LHS-> voters: {1, 2, 3}, non-voters: {}
  	// RHS-> voters: {4}, non-voters: {1, 2, 3}
  	//
  	// Remove this error path once we support swapping voters and non-voters.
  	return nil, nil,
  	errors.UnimplementedErrorf(errors.IssueLink{IssueURL: build.MakeIssueURL(58499)},
  	"unsupported configuration of RHS(%s) and LHS(%s) as it requires an atomic swap of a"+
  	" voter and non-voter", rightRepls, leftRepls)
  	}

We need to update AdminRelocateRange to use the aforementioned voter<->non-voter swaps and simply have the merge queue directly call into it, instead of having it perform its own calculation of where to move the {voter,non-voter} replicas.

[blathers-crl]

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[aayushshah15]

cc @nvanbenschoten and @andreimatei so you're aware. This fell off my radar because I didn't create a tracking issue before.