* Make propagators conform to spec
* do not modify / set an invalid span in the passed context in case
a propagator did not manage to extract
* in case no context is passed to propagator.extract assume the root
context as default so that a new trace is started instead of continung
the current active trace in case extraction fails
* fix also ot-trace propagator which compared int with str trace/span ids
when checking for validity in extract
