refactor: simplify storage

Summary

• Remove StorageWatchers, which were a lame attempt at on_replace triggers for global spaces. It'll be easy to implement something better when it's needed.
• Remove ClusterwideSpaceIndex stuff, which was added to help with storage watchers, but in practice resulted in crazy overcomplication of storage module.
• Remove ClusterwidespaceId enum, instead use a single enum ClusterwideSpace which now has methods id() and name()

This crap was added a year ago by me. At that time I was pretty sure it was a bad idea, but added it anyway semi-consciously hoping someone will stop me. But nobody did. Now I remove this code, which is overly complex and serves no purpose