Fix race condition in stream opening

[kschiffer]

Summary

This PR will address a possible race condition in the event stream logic when opening multiple subscriptions in quick succession. It also addresses an unnecessary stream closure event when switching between verbose and normal filtering.

Changes

• Make sure to send the subscription message only when the stream has been opened
• Fix wrong scoping of state variable in the stream library

Testing

• The underlying issue is somewhat hard to reproduce
• It can happen sometimes when opening a device data page and refreshing the page
• Based on how fast the connections (app events and dev events) are opened there can be a race condition where the device stream tries to send the subscripiton message before the stream was opened

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.