FYI: Remove PerModelSafeGroupState + move ConflictResolution

FYI: Remove PerModelSafeGroupState + move ConflictResolution

I started this series of patches with the intent of moving conflict resolution
to make it part of update application. I haven't figured out how to briefly
explain why this makes sense. Feel free to ask me in person if you're curious.

Along the way, it made sense to refactor the UpdateApplicator. I was
struggling to understand it while I was modifying it, so I refactored the logic
into something a bit more conventional. Part of the refactor eventually led to
the removal of ConflictProgress.

I also ended up merging the steps to Verify and Process updates. This allowed
me to remove UpdateProgress as well. We might be able to find other savings as
well; I suspec that reverifying entries is no longer necessary. However, it
turns out this part part of the change wasn't required to achieve my goal, so
it could be put back if necessary.

Finally, I was able to merge the update application and conflict resolution.

Results:
- The HasMoreToSync() loop is no longer necessary.
- Removes the last source of SYNC_CYCLE_CONTINUATION GUs, and should slightly
reduce our commit QPS, too.
- The number of syncer.cc states reduced by 3.
- The number of ModelChangingSyncerCommands per cycle reduced by 3.
- Net diff of -700 lines before making any significant changes to sync logic.
- Net diff of -1200 lines once conflict resolution was moved.
- These changes will make it much easier to implement commmit-only sync cycles.

I have no intention of committing this all at once. I intend to break them up
into separate patches, add a bit of polish, and commit them one at a time.
Consider this a road map for the next month.