An @Async method should start a new trace. Still not finished that bit
because if the user also customizes the async post processor there will
be a clash.
Biggest change here is better support for async web requests.
Each async request goes through the TraceFilter multiple times so you
have to re-attach to the span in the request if there is one.
Directly using the collector gives us more control of the annotations.
The main improvement/change here is that we need to send the span
name and parent details downstream as well as the span id. It's not
logically required by the trace model, but zipkin needs it, so it
can set annotations on the parent span (server recv/send).
