text: Title: Issue when emitting in threads
  Body: Hi,
  
  I am currently facing issues when emitting an event in a separate thread.
  
  In short:
  * Main app runs as usual
  * When task is open, I start a thread in the background
  * In the background thread, I use *flask_socketio.emit*  to emit events
  * In an Angular app I react to those events
  
  In short,
  * events from all around the app are detected
  * events from the worker thread do not work
  
  I have this issue when running the app via *socketio.run* or *eventlet.wsgi.server*.
  When using *flask run* or *gunicorn* I have no issues.
  
  Any clue on the why?
  
  I can provide a minimal example if needed.