config: make MaxValidUntilBlockIncrement configurable

[fyrchik]

Close #1962 .

[codecov]

Codecov Report

Merging #1963 (33e1e61) into master (2cb878c) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1963      +/-   ##
==========================================
+ Coverage   83.00%   83.08%   +0.07%     
==========================================
  Files         289      289              
  Lines       23667    23673       +6     
==========================================
+ Hits        19645    19668      +23     
+ Misses       2790     2770      -20     
- Partials     1232     1235       +3     

Impacted Files	Coverage Δ
pkg/core/transaction/transaction.go	88.23% <ø> (ø)
pkg/core/blockchain.go	80.08% <100.00%> (+0.10%)	⬆️
pkg/core/native/notary.go	86.58% <100.00%> (+0.05%)	⬆️
pkg/services/oracle/response.go	85.22% <100.00%> (ø)
pkg/network/server.go	70.90% <0.00%> (+0.14%)	⬆️
pkg/services/oracle/request.go	52.41% <0.00%> (+4.13%)	⬆️
pkg/services/oracle/oracle.go	90.90% <0.00%> (+11.36%)	⬆️

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