GuardMgr: Update desired bridge descs from 'update' function.

There are some TODOs left from this branch, and there's a lot of remaining code to read over and think about, but I think this will solve the biggest missing piece of the GuardMgr←→BridgeDescProvider story.

Giving review to @Diziet since they've been talking me through issues about this clown car for a few days now.

Closes #622 (closed).