Introduce extensible payloads

[fyrchik]

Close #1663 .

Sender whitelist will be done in another PR.

[codecov]

Codecov Report

Merging #1667 (b1d3ded) into master (6bc2512) will decrease coverage by 0.11%.
The diff coverage is 85.45%.

@@            Coverage Diff             @@
##           master    #1667      +/-   ##
==========================================
- Coverage   82.68%   82.56%   -0.12%     
==========================================
  Files         238      239       +1     
  Lines       19481    19540      +59     
==========================================
+ Hits        16108    16134      +26     
- Misses       2378     2402      +24     
- Partials      995     1004       +9     

Impacted Files	Coverage Δ
pkg/network/message.go	95.95% <0.00%> (ø)
pkg/network/message_string.go	33.33% <42.85%> (+3.92%)	⬆️
pkg/consensus/prepare_request.go	84.61% <75.00%> (-4.28%)	⬇️
pkg/network/server.go	71.70% <76.19%> (+0.67%)	⬆️
pkg/consensus/payload.go	83.33% <77.41%> (-9.21%)	⬇️
pkg/consensus/consensus.go	71.22% <91.11%> (+3.08%)	⬆️
pkg/consensus/recovery_message.go	83.87% <100.00%> (+0.64%)	⬆️
pkg/network/payload/extensible.go	100.00% <100.00%> (ø)
pkg/network/payload/inventory.go	100.00% <100.00%> (ø)
pkg/network/metrics/metrics.go	63.63% <0.00%> (-18.19%)	⬇️
... and 15 more

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[roman-khimov]

Please merge dbft (and update its revision here) and this will be good to merge.