Implement unscubscribe on event stream

[stana-miric]

Description

This PR implements unsubscribe on event stream, that will be called when subscription is closed. Earlier event stream held slice of update channels which belonged to subscriptions and were used to notify subscription when the event is received (push fn). The problem here was if some subscription is closed, its update channel was not removed from the event stream, and since it is buffered channel of 5 events, after the buffer was full and events kept coming, push function was blocked on writing to that channel preventing other subscriptions to receive events. Earlier there was default branch, but we removed it to avoid loosing events.
With this fix events stream is changed to have map of subscription and when unsubscribe is called on event stream that will remove the subscription and close subscription channel.

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