Scans each storage only once when building accounts index

[brooksprumo]

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.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.3%. Comparing base (b979967) to head (a929105).
⚠️ Report is 3054 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6745   +/-   ##
=======================================
  Coverage    83.3%    83.3%           
=======================================
  Files         852      852           
  Lines      378005   378020   +15     
=======================================
+ Hits       315220   315251   +31     
+ Misses      62785    62769   -16     

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[HaoranYi]

nice find.
lgtm.

[mergify]

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