Uses IntSet for RemoveUnrootedSlotsSynchronization::slots_under_contention

[brooksprumo]

Problem

Accounts DB keeps a list of slots that is shared between flush and purge. This list is behind a mutex to ensure the two background processes don't interfere with each other. This list of slots is currently a HashSet, but we don't need strong cryptographic hashing on the slot; a simple vector would be fine too. We can use an IntSet to use the Slot directly, bypassing the need to perform an expensive hash.

Summary of Changes

Use IntSet for RemoveUnrootedSlotsSynchronization::slots_under_contention.

[HaoranYi]

lgtm

[codecov]

Codecov Report

Merging #33530 (034de59) into master (f714a44) will decrease coverage by 0.1%.
Report is 2 commits behind head on master.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           master   #33530     +/-   ##
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- Coverage    81.7%    81.7%   -0.1%     
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  Files         805      805             
  Lines      218111   218111             
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- Hits       178360   178351      -9     
- Misses      39751    39760      +9