Currently, ShardingTest calls stopSet on each of its shard ReplSetTest instances serially. This means that it is not possible to be stopping processes from multiple shards at the same time. To make this stop procedure faster when there are many shards, ShardingTest can stop all ReplSetTest instances at the same time, without waiting for one to complete before moving on to the next one. It can then wait for all shard stop procedures to finish. This will allow the stopping of all shard ReplSetTest instances to proceed in parallel.