In the absence of an SDK the API should not record spans anymore but it
should propagate the span context. In our case absence of an SDK means
we're using the `NoopTracer`.

There are different ways to create new spans and they provide different
ways of specifying the parent context:

- Tracer::start: uses Context::current()
- Tracer::build: uses the context from the span builder or falls back to
  the current context. BUT it did not have the fallback in the
  NoopTracer.
- Tracer::start_with_context: uses the specified context

This changes the `SpanBuilder` to make the `parent_context` in it
non-optional. That way the current context fallback is done on
`SpanBuilder` creation. Tracer implementations just need to read the
`parparent_context` from the span builder.