check: set lastBlockTime in PrepareRequest handler, fix #55

[roman-khimov]

It's the earliest point, if a view change happens the timer is reset (because
new view comes with a new set of transactions, potentially picking up ones
received between views).

[roman-khimov]

Four nodes, 1000 TPS:

Same with 200ms network delay:

[roman-khimov]

Max performance for four nodes with 5s block time:

With 200ms delay:

I'm not entirely sure why, but does affect TPS and not in a good way (hence this is a draft).

[roman-khimov]

Two seconds with 128K pool:

[roman-khimov]

Rebased.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.35%. Comparing base (96105d0) to head (4c7e221).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   63.31%   63.35%   +0.04%     
==========================================
  Files          27       27              
  Lines        1510     1512       +2     
==========================================
+ Hits          956      958       +2     
  Misses        489      489              
  Partials       65       65              

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