Add pool for Extensible payloads

[fyrchik]

We should store them in one place.
Close #1663 .

[codecov]

Codecov Report

Merging #1674 (b83dc31) into master (32e8678) will decrease coverage by 0.02%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master    #1674      +/-   ##
==========================================
- Coverage   82.66%   82.63%   -0.03%     
==========================================
  Files         239      240       +1     
  Lines       19546    19635      +89     
==========================================
+ Hits        16157    16226      +69     
- Misses       2392     2403      +11     
- Partials      997     1006       +9     

Impacted Files	Coverage Δ
pkg/consensus/consensus.go	69.57% <ø> (-1.07%)	⬇️
pkg/core/blockchain.go	76.04% <64.10%> (-0.51%)	⬇️
pkg/network/server.go	72.10% <64.70%> (+0.56%)	⬆️
pkg/core/native/designate.go	74.48% <75.60%> (-2.94%)	⬇️
pkg/core/native/oracle.go	81.55% <100.00%> (ø)
pkg/network/extpool/pool.go	100.00% <100.00%> (ø)
pkg/rpc/client/wsclient.go	82.70% <0.00%> (+1.50%)	⬆️

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[fyrchik]

I don't like how whitelist is cached, but haven't come up with anything more simple. It would be good if whitelist was stored in the pool only, but it's calculation depends on native contract state.

[roman-khimov]

Rebase it please to only leave relevant commits.

[roman-khimov]

Can we improve coverage to 80%+?

[roman-khimov]

Tests fail here.

[roman-khimov]

Conflicts.