In the early days Elasticsearch used to use the index name as the index identity. Around 1.0.0 we introduced a unique index uuid which is stored in the index setting. Since then we used that uuid in a few places but it is by far not the main identifier when working with indices, partially because it's not always readily available in all places.
This PR start to make a move in the direction of using uuids instead of name by making sure that the uuid is available on the Index class (currently just a wrapper around the name) and as such also available via ShardRouting and ShardId.
Note that this is by no means an attempt to do the right thing with the uuid in all places. In almost all places it falls back to the name based comparison that was done before. It is meant as a first step towards slowly improving the situation.
Closes#16217
2a137b5548