Recent versions of zipkin and brave operate with http instead of scribe.
This changes the implementation accordingly, as well adds a sampler to
the POST endpoint.
A notable impact is that we no longer require a collector process, as
the zipkin server's POST endpoint is a collector.
Instrumentation should be able to get by with only 2
interfaces: TraceManager and TraceAccessor (the former is
not needed if you aren't starting a new Span). No explicit
access to thread locals or manipulation of thread context
is required (except locally where necessary).
A Span is enclosed by a Trace (actually a view of the complete
Trace that would be constructed remotely).
