Updates the way vip-search versioning information is stored

The versioning information for vip-search index needed to be updated to address global indexables (e.g. User), and different versioning across sites in a multisite environment.

This change stores global indexable separately – one per network. While making sure non-global versions are stored one per site.

https://github.com/Automattic/vip-go-mu-plugins/pull/1921

#mu-plugins #search