W3C span propagation id fix

[SimunKaracic]

Propagate the current span id, not the parent span id, according to the spec.

When reading from headers, don't generate a new ID for the Remote span, but use the existing one.

This is the thread where @dguggemos reported the bug -> https://gitter.im/kamon-io/Kamon?at=60cc4e2cb31731135437e0aa

@pnerg If you have the time, check this out. You initially changed this to use the parentID instead of the spanID. According to the W3C spec, the current span ID should be propagated.
From the spec https://www.w3.org/TR/trace-context/#a-traceparent-is-received:
Update parent-id: The value of property parent-id MUST be set to a value representing the ID of the current operation.

#986 I believe this issue originated from the fact that akka-http was being double instrumented.

[the-overengineer]

We talked about this a bit offline, looks good to me after looking at the spec. Please make sure this doesn't do something terribly wrong to Zipkin (or we at least need to check whether we need to adjust the joining of spans flag in the config/document it), though. The Span.Remote is just a reference with the same ID, so this should work, but issues might arise due to only one of the sides having the correct parent span ID.

[SimunKaracic]

There's only one issue. When using w3c, we need to make sure that kamon.trace.identifier-scheme is set to double. This is already documented in the configuration, but we should probably warn if it's set to single. I'll try and get it done now, but if it's too complicated I'll create a task for it for the future