[Memory fix] Remove memory leak in event subscriptions

[zivkovicmilos]

Description

This PR removes the memory leak present in the blockchain event subscriptions.

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

Manual tests

Tested this out with the monitor command.

Additional comments

Fixes EDGE-849

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release/0.5.1@68df30d). Click here to learn what that means.
The diff coverage is n/a.

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[0xAleksaOpacic]

LGTM