state: Improve account insert/get API

[chfast]

No description provided.

[codecov]

Codecov Report

Merging #519 (8314df3) into master (6937f5d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #519      +/-   ##
==========================================
+ Coverage   99.06%   99.08%   +0.01%     
==========================================
  Files          54       54              
  Lines        5352     5348       -4     
==========================================
- Hits         5302     5299       -3     
+ Misses         50       49       -1     

Flag	Coverage Δ
blockchaintests	77.78% <ø> (ø)
statetests	7.56% <100.00%> (-0.07%)	⬇️
unittests	97.26% <0.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
test/state/account.hpp	100.00% <ø> (ø)
test/state/state.hpp	100.00% <100.00%> (+20.00%)	⬆️
test/statetest/statetest_loader.cpp	80.86% <100.00%> (-0.49%)	⬇️