Add digest re-check before removing followers in synchronization mechanism

[ClearlyClaire]

The follower synchronization mechanism works by computing a digest of follow the expected followers of an account on a remote server, then fetching that collection if needed.

Currently, Mastodon does not re-check the expected digest on the newly-fetched collection, as it assumes any change may just be down to the newly-fetched collection having more up-to-date values.

This PR adds re-checking of the digest before removing any follower, potentially guarding against stale caches and possible other errors, although I'm not sure when this would actually occur.

This also requires that the partial collection features no duplication, even across pages.

Benefits

• may guard against some unlikely classes of implementation issues
• may guard against wrongly undoing recent follows in case of stale cached collections

Drawbacks

• increased code complexity
• increased computation
• requires the collection to not have duplicates across pages
• may cause follower synchronization to be skipped if too much time occurs between receiving a hash and processing the collection

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.

[github-actions]

This pull request has resolved merge conflicts and is ready for review.