[Memory leak] Remove dangling pointers in the blockchain subscription

[DarianShawn]

Description

The blockchain subscription hold the whole update events and never release it, which made a memory leak.

This PR fixes it. It is merging from 0xPolygon Edge project PR 741.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

[codecov-commenter]

Codecov Report

Merging #194 (0dc4378) into dev (c648f03) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #194      +/-   ##
==========================================
+ Coverage   51.37%   51.39%   +0.01%     
==========================================
  Files         114      114              
  Lines       16986    16962      -24     
==========================================
- Hits         8727     8718       -9     
+ Misses       7519     7504      -15     
  Partials      740      740              

Impacted Files	Coverage Δ
blockchain/subscription.go	81.57% <100.00%> (+9.57%)	⬆️
protocol/sync_peer.go	70.00% <0.00%> (+1.00%)	⬆️

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[0xcb9ff9]

LGTM