core,rpc: implement NEP5-related logic

[fyrchik]

1. Track NEP5 balances and transfer events.
2. Implement getnep5balances and getnep5transfers RPC.

Closes #498 .

[codecov]

Codecov Report

Merging #723 into master will increase coverage by 0.26%.
The diff coverage is 67.5%.

@@            Coverage Diff             @@
##           master     #723      +/-   ##
==========================================
+ Coverage   66.47%   66.73%   +0.26%     
==========================================
  Files         134      135       +1     
  Lines       11895    12127     +232     
==========================================
+ Hits         7907     8093     +186     
- Misses       3625     3643      +18     
- Partials      363      391      +28

Impacted Files	Coverage Δ
pkg/core/storage/store.go	37.5% <ø> (ø)	⬆️
pkg/rpc/server/prometheus.go	100% <ø> (ø)	⬆️
pkg/rpc/client/rpc.go	0% <0%> (ø)	⬆️
pkg/smartcontract/parameter.go	74.89% <100%> (-0.31%)	⬇️
pkg/core/state/account.go	58.75% <23.07%> (-6.93%)	⬇️
pkg/core/blockchain.go	46.44% <59.25%> (+1.47%)	⬆️
pkg/core/dao.go	70.24% <65%> (-0.35%)	⬇️
pkg/rpc/server/server.go	83.97% <80.41%> (-0.7%)	⬇️
pkg/core/state/nep5.go	80.48% <80.48%> (ø)
... and 2 more

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