SERVER-46199 introduces a causally consistent catalog cache, which, among other consistency improvements, also optimizes the number of round trips to the config server after detecting a router is stale and needs to advance the local shard version.

Another improvement can be achieved by returning the resulted collection version on the split chunk and merge chunks commands, because instead of fully invalidating the cache after splitting or merging, we could simply advance the shard version which in turn would only request the diff of the changes instead of a full chunk request.