Propagate opentracing contexts across workers
Also includes some Convenience modifications to opentracing for servlets, notably:
- Add boolean to skip the whitelisting check on inject
extract methods. - useful when injecting into carriers
locally. Otherwise we'd always have to include our
own servername and whitelist our servername
- start_active_span_from_request instead of header
- Add boolean to decide whether to extract context
from a request to a servlet
