Right now the mock holds a single list of

pairs that it must see in order. This was fine before the sharded facets and search in $lookup projects as search requests would never issues commands across shards. However now that this is possible, the mock should hold a set of lists of pairs, each one marking the order of requests for a given shard as opposed to forcing the total set to be ordered. This will allow the tests to pass without depending on the shards being exactly in sync.