Add test FlushableDB iterator: TestFlushableParallelCh()

[devintegral2]

Добавлен новый тест в которм испралена ситуация с состоянием гонки. При различных параметрах баг не проявлялся.

[codecov-io]

Codecov Report

Merging #484 into develop will decrease coverage by 0.02%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #484      +/-   ##
===========================================
- Coverage    47.56%   47.53%   -0.03%     
===========================================
  Files          189      189              
  Lines        13812    13812              
===========================================
- Hits          6570     6566       -4     
- Misses        6670     6673       +3     
- Partials       572      573       +1     

Impacted Files	Coverage Δ
evmcore/tx_pool.go	74.44% <0.00%> (-0.80%)	:arrow_down:
inter/ascii_scheme.go	83.89% <0.00%> (+0.56%)	:arrow_up:

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[devintegral]

Outdated, see #500