improve W3C Trace Context compliance

[reyang]

redo PR #931

Trying to improve the compliance by fixing:

1. Current version allows trace options to be optional, which is against https://github.com/w3c/distributed-tracing/blob/master/trace_context/HTTP_HEADER_FORMAT.md#traceparent-field.
2. Current implementation doesn't combine multiple headers with the same name.
3. Version compatibility.

[rghetia]

From spec perspective it looks good.

/cc @Ramonza . He had some concern with go style.

[reyang]

@Ramonza would you help to review?