Uses scan_accounts() while generating the secondary indexes

[brooksprumo]

Problem

When generating secondary account indexes, we scan storages and pass each account to update_secondary_indexes(). This function needs a pubkey and a ReadableAccount per account in the storage, but currently we load/pass around the whole StoredAccountMeta. This struct should be avoided when possible, as it contains AppendVec file format implementation details. And since we don't need any of those file format details, we should use a different function for scanning the storages.

Summary of Changes

Uses scan_accounts() while generating the secondary indexes.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.9%. Comparing base (679ad80) to head (4dea142).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #5779     +/-   ##
=========================================
- Coverage    83.0%    82.9%   -0.1%     
=========================================
  Files         826      826             
  Lines      375839   375836      -3     
=========================================
- Hits       311955   311890     -65     
- Misses      63884    63946     +62     

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[HaoranYi]

lgtm.