v2.3: Scans each storage only once when building accounts index (backport of #6745)

[mergify]

Problem

When building the accounts index, if secondary indexes are on, we scan each account storage file twice. This is bad, performance wise. The second scan is needed to get each account's data, which secondary indexes need.

We should switch on if secondary indexes are enabled or not, and then decide how to scan the storage. E.g. if secondary indexes are on, scan once with account data. If secondary indexes are off, scan once without account data.

Summary of Changes

Instead of scanning twice, only scan once! Pick the scan function depending on if secondary indexes are on or off.

Justification to Backport

@alessandrod asked me to. It is for startup perf improvement when secondary indexes are used (rpc-only).

---

This is an automatic backport of pull request #6745 done by [Mergify](https://mergify.com).

[brooksprumo]

Note, these two fields (rent_epoch and executable) were removed from IndexInfoInner in master. They are not used here.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.8%. Comparing base (9963221) to head (9c3689b).

Additional details and impacted files

@@           Coverage Diff           @@
##             v2.3    #6896   +/-   ##
=======================================
  Coverage    82.8%    82.8%           
=======================================
  Files         850      850           
  Lines      379824   379841   +17     
=======================================
+ Hits       314710   314744   +34     
+ Misses      65114    65097   -17     

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[HaoranYi]

lgtm.