Don't hold storage accounts lock during scan

[carllin]

Problem

Accounts scans from RPC hold:

1. the account storage lock duration the entire duration of the scan, which blocks replay on create_and_insert_store() during account commit.

2. the account index lock duration the entire duration of the scan, which blocks replay in AccountsDb store()->update_index() during account commit.

Summary of Changes

Addresses problem 1). Release storage lock after each account lookup in scan.

Compared to #12126:

Pros: Is less disruptive

Cons:

1. is less efficient, need to repeatedly grab storage lock,
2. storage lock lookup still contends briefly with storage lock write in create_and_insert_store()
3. Doesn't generalize as easily to also solve problem 2) the accounts index lock above, need to handle this problem separately

Fixes #

[codecov]

Codecov Report

Merging #12132 into master will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master   #12132   +/-   ##
=======================================
  Coverage    82.0%    82.0%           
=======================================
  Files         337      337           
  Lines       79249    79250    +1     
=======================================
+ Hits        65024    65031    +7     
+ Misses      14225    14219    -6     

[stale]

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