The ChunkInfo already contains shardRefs after SERVER-127412, but the chunk manager is converting this to shardId. Changing this to a shard ref in the chunk manager will allow a lot of our routing code to be converted to shardref.

Note that this will need to include some magic within the chunk manager to ensure that a chunk manager with a mix of shard ids and shard uuids can still function correctly (ie. a caller asking for chunks on a shard sees all of the chunks and not just those with the uuid or the shardid matching the input).