Remove possibility to nest client spans.

This was added as a pull request earlier but I don’t think it is the
future of Brave. I think we should stick to RPC tracing. See discussion
here: https://groups.google.com/forum/#!topic/zipkin-user/a80ZQqoJpWg
Update client interceptor to set service name.

This will make sure client and server service name are equal. Results
in better representation in zipkin-web.
Extend integration test to check more than just equal span names.