Span lifetime / active span state

[jtescher]

Currently spans are stored in a thread local span stack, and either manually added and removed via mark_span_as_active / mark_span_as_inactive or for the scope of a given closure by using with_span.

This has certain drawbacks: spans marked as active and not later marked as inactive will not be dropped, marking as inactive is unexpected as it is not part of the spec, etc.

We should investigate if there are more reliable / intuitive / ergonomic ways of coordinating active span information between components. E.g. web framework middleware needing to set the active span for use later in the application.

[iredelmeier]

See also #15 (I think they're related but separate)

[jtescher]

Closing this in favor of #15