This enhancement intends to refine PropagationContextElement by:

• Capturing the ThreadLocal when instantiating the PropagationContextElement in order to support dispatchers switching threads
• Removing the constructor parameter which is not idiomatic and breaks the support when switching threads, and use instead the updateThreadContext(context: CoroutineContext) parameter
• Making the kotlinx-coroutines-reactor dependency optional
• Making the properties private
• Documentation the recommended usage as Dispatchers.IO + PropagationContextElement()

cc @bclozel