…ting tests

- continueTrace now returns a TransactionContext making it easier to
  create a transaction from incoming headers
- TransactionContext now has defaults for name, source and op and
  setters for them
- deprecate traceHeaders() and replace with getTraceparent()
- rename baggageHeader() to getBaggage()
- getBaggage() no longer takes pre existing headers
- add trace context to envelope payload from scope if none present and
  no span on scope
- improve JavaDoc
- create new PropagationContext when cloning scope

…cit sampling decision

…ound