Set watermark to range end_ts for latency improvement.

[claudevdm]

Previous watermark was held back to the start of the query range which was overly conservative and causing data to be delayed by poll_duration.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.15%. Comparing base (2b78140) to head (a846e96).
⚠️ Report is 29 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (2b78140) and HEAD (a846e96). Click for more details.

HEAD has 8 uploads less than BASE

Flag	BASE (2b78140)	HEAD (a846e96)
	9	1

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #3912       +/-   ##
=============================================
- Coverage     55.52%   38.15%   -17.38%     
+ Complexity     7018      510     -6508     
=============================================
  Files          1103      195      -908     
  Lines         67557    11697    -55860     
  Branches       7576     1167     -6409     
=============================================
- Hits          37512     4463    -33049     
+ Misses        27626     6877    -20749     
+ Partials       2419      357     -2062     

Components	Coverage Δ
spanner-templates	∅ <ø> (∅)
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	∅ <ø> (∅)
spanner-live-reverse-replication	∅ <ø> (∅)
spanner-bulk-migration	∅ <ø> (∅)
gcs-spanner-dv	∅ <ø> (∅)
see 935 files with indirect coverage changes

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