Fix event stream concurrency and state machine

[kschiffer]

Summary

This is hopefully the last fix to the event stream logic. It adds a proper state machine store logic and addresses various state edge cases.

Changes

• Store subscriptions properly per connection (including ws instaces)
• Add state machine logic to read on modify from the currently stored connections and subscriptions

Testing

Manual testing on staging on Chrome and Firefox

Notes for Reviewers

Please test various scenarios:

• Testing in different browsers
• Testing multiple subscriptions (when checking device events)
• Testing interruption of the stream (eg. CTRL-C the stack)
• Refreshing many times

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• The steps/process to test this feature are clearly explained including testing for regressions.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.