config, opt: teach localityMatchScore about voter_constraints

[aayushshah15]

localityMatchScore() aids in computing the scan cost of a single row, from a given index idx, based on how well the locality tiers of the gateway match the zone config constraints on idx. This allows the cost model to prioritize choosing indexes that are geographically closer over the ones that are further away from the gateway node.

With the introduction of the new voter_constraints attribute, voting replicas are constrained by the existing constraints (which apply to all replicas - voting and non-voting) as well as voter_constraints (which apply only to voting replicas). Thus, localityMatchScore() needs to be updated to include this new attribute when computing its result.

Jira issue: CRDB-3285

[rytaft]

cc @rmloveland I've added docs-known-limitation, since until this issue is addressed, the duplicate indexes pattern won't work with "voter constraints" in zone configs. The suggested workaround is to use GLOBAL tables instead of duplicate indexes if users want fast reads and non-voting replicas.

(Existing schemas with the duplicated indexes pattern should work fine -- what won't work is if users try to update the zone configs to use the new "voter constraints" instead of "constraints")

[rmloveland]

Thanks Becca! Filed cockroachdb/docs#10173