[ASTS] Background Shard Progress Updater now correctly calculates last swept progress

[mdaudali]

General

Before this PR:
See #7439

After this PR:
Background Shard Progress Updater now calculates the correct timestamp in the presence of deleted progress entries

==COMMIT_MSG==
==COMMIT_MSG==

Priority: P2

Concerns / possible downsides (what feedback would you like?):
The old code set knownSweepProgress to endExclusive + 1. I'm pretty sure it's endExclusive - 1, since endExclusive hasn't been swept (only upto endExclusive has been swept)

-1 as a placeholder is janky. Happy to move to Optional, but given it's scoped down to within a method and not used as a return value, I figured that not unpacking the optional was easier to read.
Is documentation needed?:
N/A

Compatibility

Does this PR create any API breaks (e.g. at the Java or HTTP layers) - if so, do we have compatibility?:
no
Does this PR change the persisted format of any data - if so, do we have forward and backward compatibility?:
no
The code in this PR may be part of a blue-green deploy. Can upgrades from previous versions safely coexist? (Consider restarts of blue or green nodes.):
yes
Does this PR rely on statements being true about other products at a deployment - if so, do we have correct product dependencies on these products (or other ways of verifying that these statements are true)?:
no
Does this PR need a schema migration?
n/a

Testing and Correctness

What, if any, assumptions are made about the current state of the world? If they change over time, how will we find out?:
Not rolled out
What was existing testing like? What have you done to improve it?:
Fixed existing and added new tests
If this PR contains complex concurrent or asynchronous code, is it correct? The onus is on the PR writer to demonstrate this.:
N/A
If this PR involves acquiring locks or other shared resources, how do we ensure that these are always released?:
N/A

Execution

How would I tell this PR works in production? (Metrics, logs, etc.):
Shard Progress Updater makes progress and updates last swept metric
Has the safety of all log arguments been decided correctly?:
N/A
Will this change significantly affect our spending on metrics or logs?:
N/A
How would I tell that this PR does not work in production? (monitors, etc.):
Shard Progress Updater still doesn't work
If this PR does not work as expected, how do I fix that state? Would rollback be straightforward?:
Fix it
If the above plan is more complex than “recall and rollback”, please tag the support PoC here (if it is the end of the week, tag both the current and next PoC):
N/A

Scale

Would this PR be expected to pose a risk at scale? Think of the shopping product at our largest stack.:
No
Would this PR be expected to perform a large number of database calls, and/or expensive database calls (e.g., row range scans, concurrent CAS)?:
No
Would this PR ever, with time and scale, become the wrong thing to do - and if so, how would we know that we need to do something differently?:
If too many buckets are being processed between iterations - but at 10 minutes per bucket for creation, that's unlikely. If we move to a world where we're creating lots of tiny buckets, then it's different.

Development Process

Where should we start reviewing?:
DSPU
If this PR is in excess of 500 lines excluding versions lock-files, why does it not make sense to split it?:

Please tag any other people who should be aware of this PR:
@jeremyk-91
@raiju

[mdaudali]

See concern - I'm pretty sure the old + 1 was wrong?

[mdaudali]

Not necessary for this PR, but handles the case for more buckets than limit which existing tests don't (and surfaces the -1 / +1 error)