Fix notifications and getreferences bugs

[roman-khimov]

Problem

#454, #453.

Solution

"Merge" button.

[codecov]

Codecov Report

Merging #499 into master will decrease coverage by 0.35%.
The diff coverage is 12.19%.

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
- Coverage   53.41%   53.06%   -0.36%     
==========================================
  Files         116      117       +1     
  Lines        8529     8592      +63     
==========================================
+ Hits         4556     4559       +3     
- Misses       3644     3704      +60     
  Partials      329      329

Impacted Files	Coverage Δ
pkg/core/storage/store.go	37.5% <ø> (ø)	⬆️
pkg/core/blockchain.go	38.7% <0%> (-1.46%)	⬇️
pkg/core/notification_event.go	0% <0%> (ø)
pkg/core/interop_neo.go	0% <0%> (ø)	⬆️
pkg/vm/stack.go	87.69% <0%> (-0.91%)	⬇️
pkg/core/interop_system.go	0% <0%> (ø)	⬆️
pkg/vm/stack_item.go	62.58% <0%> (ø)	⬆️
pkg/core/interops.go	100% <100%> (ø)	⬆️
pkg/vm/serialization.go	95.4% <100%> (+0.1%)	⬆️

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