Bring up to date the concurrent accounts benches

[brooksprumo]

Problem

The Accounts Index has a type to represent reading an entry that is internally self referencing (ReadAccountMapEntry). In #34786, this type has been identified as contributing significantly to memory management overhead. That issue points to #12787 as the source of the type, and this pull-request mentions (and adds code for) benches that justify the changes.

However, running these benches fails because it violates assertions in accounts db:

thread 'main' panicked at accounts-db/src/account_storage.rs:137:9:
assertion failed: self.map.insert(slot,
        AccountStorageReference {
            id: store.append_vec_id(),
            storage: store,
        }).is_none()

This is because we now always use a write cache in AccountsDb, and these benches were written before the write cache was required.

I'd like to use these benches while working on #34786, so these benches need to be updated to run again.

Summary of Changes

Update store_accounts_with_possible_contention() such that it stores accounts using the write cache.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0e8f2de) 81.7% compared to head (e72887d) 81.7%.
Report is 136 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34815   +/-   ##
=======================================
  Coverage    81.7%    81.7%           
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  Files         825      825           
  Lines      223249   223250    +1     
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+ Hits       182609   182610    +1     
  Misses      40640    40640           

[jeffwashington]

lgtm
all in 'benches', so no runtime impact