Replaces ReadAccountMapEntry in do_scan_secondary_index()

[brooksprumo]

Problem

See https://github.com/solana-labs/solana/issues/34786 for background.

We want to limit the use of ReadAccountMapEntry, from AccountsIndex, everywhere. Ultimately removing it once there are no more uses.

When scanning a secondary index, we look up each pubkey in the index and then apply a callback function to the entry if found. Currently this uses ReadAccountMapEntry via AccountsIndex::get(), but we can do it other ways without ReadAccountMapEntry.

Summary of Changes

Adds get_with_and_then() and uses it in do_scan_secondary_index()

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.7%. Comparing base (da08868) to head (604162f). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #35219     +/-   ##
=========================================
- Coverage    81.7%    81.7%   -0.1%     
=========================================
  Files         834      834             
  Lines      224244   224249      +5     
=========================================
- Hits       183398   183381     -17     
- Misses      40846    40868     +22     

[brooksprumo]

I rebased and force-pushed this branch to pull in #35307.

[brooksprumo]

@HaoranYi @jeffwashington This PR is now ready for a re-review.

[brooksprumo]

Rebased on top of #35336, so that I can use get_account_info_with_and_then() in the impl of this PR.

[brooksprumo]

@HaoranYi @jeffwashington This PR is ready for re-review. Thanks!