remove multi slot test only StorableAccounts impl

[jeffwashington]

Problem

stop mmapping storage files.
For shrink operations, we no longer hold a ref to account data. We have to lookup the account when we are creating new storages. In the naive implementation, we have to lookup the storage per slot once per account. We can do better by keeping a cache of the last storage lookup while writing. This is most easily done as part of StorableAccountsBySlot.

Summary of Changes

remove the test-only StorableAccounts impl that also tried to load accounts at store time. This will facilitate optimization of the validator use case.

Fixes #

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.9%. Comparing base (b745dc9) to head (acfb70c).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #972     +/-   ##
=========================================
- Coverage    81.9%    81.9%   -0.1%     
=========================================
  Files         853      855      +2     
  Lines      231955   232097    +142     
=========================================
+ Hits       189984   190094    +110     
- Misses      41971    42003     +32