Adds LocalTracer, designed for in-process activity that explains latency

[codefromthecrypt]

A local span can represent bootstrap, codec, file i/o or other activity
that notably impacts performance.

Local spans always have a binary annotation "lc" which indicates the
component name. Usings zipkin's UI or Api, you can query by for spans
that use a component like this: lc=spring-boot

Here's an example of allocating precise duration for a local span:

tracer.startSpan("codec", "encode");
try {
  return codec.encode(input);
} finally {
  tracer.finishSpan();
}

Fixes #111
See openzipkin/zipkin#821

Another example: Local span where user does know about time and duration, such as recording events that already occurred:

if (localTracer.startSpan("spring-boot", "bootstrap", startTimestamp) != null) {
  // log historical annotations/binary annotations only if we know the trace was sampled
  localTracer.finishSpan(duration);
}

[codefromthecrypt]

@kristofa PTAL I'm done I think :)

[kristofa]

@adriancole Thank you! LGTM!