Cleanup SnapshotsInProgress

* Added separate enum for the state of each shard, it was really
confusing that we used the same enum for the state of the snapshot
overall and the state of each individual shard
   * relates elastic#40943 (comment)
* Moved the contents of the class around a little so fields,
constructors and nested classes/enums aren't all over the place
especially now that we have yet another nested enum here
* Shortened some obvious spots in equals method and saved a few lines
via `computeIfAbsent` to make up for adding 50 new lines to this class