Implement getutxotransfers RPC (2.x)

[fyrchik]

Reuse NEP5TransferLog for everything.

[codecov]

Codecov Report

Merging #1268 into master-2.x will decrease coverage by 0.20%.
The diff coverage is 54.27%.

@@              Coverage Diff               @@
##           master-2.x    #1268      +/-   ##
==============================================
- Coverage       67.93%   67.73%   -0.21%     
==============================================
  Files             159      160       +1     
  Lines           15601    15876     +275     
==============================================
+ Hits            10599    10754     +155     
- Misses           4478     4583     +105     
- Partials          524      539      +15     

Impacted Files	Coverage Δ
pkg/core/dao/dao.go	39.61% <0.00%> (-4.12%)	⬇️
pkg/core/storage/store.go	34.61% <ø> (ø)
pkg/core/dao/cacheddao.go	30.83% <27.58%> (-0.39%)	⬇️
pkg/core/blockchain.go	45.54% <50.00%> (+0.34%)	⬆️
pkg/core/state/transfer_log.go	50.00% <50.00%> (ø)
pkg/core/state/nep5.go	52.94% <85.71%> (-0.69%)	⬇️
pkg/rpc/server/server.go	84.01% <97.89%> (+1.49%)	⬆️
pkg/rpc/client/wsclient.go	85.71% <0.00%> (+1.36%)	⬆️

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