Better Incrementality for Snapshots of Unchanged Shards

[original-brownbear]

Use sequence numbers to determine whether a shard has changed or not instead before falling back to comparing files to get incremental snapshots on primary fail-over.

[original-brownbear]

@ywelsch @dnhatn Thanks for all the help here so far:

I incorporated feedback and the work in #52694 into this PR now. Instead of dealing with all the details of the global- and local-checkpoints being equal etc. in the repository this is now all happening upstream in the SnapshotShardsService.
The repository now simply accepts a string identifier for a shard's state that is then checked against existing file sets. The identifier is then either null if we don't have local == global checkpoint or else a string made up from the history- and forcemerge uuids and the current checkpoint (that should be unique in all cases I think).

=> I think this one is good for another round of reviews whenever you have some time :)

[ywelsch]

I've left one ask for a test, o.w. looking good.

[ywelsch]

Can you also add a test that checks that force-merging is leading to another full snapshot?

[original-brownbear]

Sure thing, how about this one b508fb3

[ywelsch]

LGTM

[original-brownbear]

Thanks Yannick!