Returns IntMap from select_candidates_by_total_usage()

[brooksprumo]

Problem

AccountsDb::select_candidates_by_total_usage() returns a HashMap of slots. Since this function (and return value) is an implementation detail of shrink, it is not part of consensus, and its key (Slot) does not need to be cryptographically secure. So we are free to change it out to be something faster.

Since the Slot itself is a sufficient unique identifier, we can use it directly as the hashing function. This is built in as part of IntMap.

Summary of Changes

Returns IntMap from select_candidates_by_total_usage()

[codecov]

Codecov Report

Merging #33976 (fc09fe8) into master (8c5b5f1) will increase coverage by 0.0%.
Report is 10 commits behind head on master.
The diff coverage is 100.0%.

❗ Current head fc09fe8 differs from pull request most recent head e67bbd6. Consider uploading reports for the commit e67bbd6 to get more accurate results

@@           Coverage Diff           @@
##           master   #33976   +/-   ##
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  Coverage    81.9%    81.9%           
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  Files         811      811           
  Lines      219354   219354           
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+ Hits       179749   179756    +7     
+ Misses      39605    39598    -7     

[HaoranYi]

lgtm