shrink accounts index after eviction

[HaoranYi]

Problem

If account's index map overgrows, we never shrink it even when later on the map become smaller.

Summary of Changes

Shrink the account's map when it is less than half used.

Fixes #

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.2%. Comparing base (b210780) to head (a66bde8).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #8823     +/-   ##
=========================================
- Coverage    83.2%    83.2%   -0.1%     
=========================================
  Files         864      864             
  Lines      375150   375150             
=========================================
- Hits       312224   312203     -21     
- Misses      62926    62947     +21     

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[brooksprumo]

Let's not do this. IMO we shouldn't shrink because we'll likely need to grow shortly after, assuming that accounts keep being created. So we'd end up paying to move/reallocate.

I think when we have a properly memory limit for the in-mem index that we'll be good.