rework remove_dead_accounts.reclaimed_offsets

[jeffwashington]

Problem

clean is inefficient and can be very slow.
A big offender is remove_dead_accounts.
This fn could be optimized by using a HashMap that organizes the requests by slot. We already had an Optional way to do that. We would like to always do that.

Summary of Changes

Always build reclaimed_offsets HashMap.

Fixes #

[brooksprumo]

Refactor looks good to me.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (0168e0a) to head (bd85ab8).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #571     +/-   ##
=========================================
- Coverage    81.8%    81.8%   -0.1%     
=========================================
  Files         847      849      +2     
  Lines      229180   229190     +10     
=========================================
- Hits       187600   187551     -49     
- Misses      41580    41639     +59