Preserve key order of #fetch_multi

[gmcgibbon]

Summary

Fixes #34694.

Preserves order of keys passed to #fetch_multi.

If for some reason the current functionality is intended, I'll refactor this PR to document the behaviour.

[jeremy]

Nice @gmcgibbon!

[jeremy]

Let's illuminate this a bit, e.g. fetch_multi(*names) returns its results in the same order as the *names requested, rather than returning cache hits followed by cache misses.

[kaspth]

Tried grokking this over multiple attempts, but I find the two maps quite confusing since we seem to do similar yet different processing.

What if we kept some of the old flow ala this?

instrument :read_multi, names, options do |payload|
  reads = read_multi_entries(names, options)
  payload[:hits] = reads.keys
  payload[:super_operation] = :fetch_multi

  writes  = {}
  ordered = names.each_with_object({}) do |name, hash|
    hash[name] = reads.fetch(name) { writes[name] = yield name }
  end

  write_multi writes, options
  ordered
end

(Probably ends up needing to be yield(name). Psst, also a nice case for index_with, but we can't use core extensions here.)