Missing service name:
Zipkin service names were logged as null, which is invalid and led to
them showing up as "unknown" in the zipkin ui. This was due to a wiring
bug, and a special-case, which this change fixes.
The special-case was when a sleuth span had no annotations. Since zipkin
service names are attached to annotations, they are only queryable when
annotations exist. When there are no annotations, we add the "lc"
binary annotation, which makes that span attached to the correct service
in zipkin.
Polishing:
Zipkin timestamps were not always set as microseconds. This fixes that.
The de-facto label in zipkin for unknown service is "unknown". This
fixes the code, which formerly fell back to "application".
This also removes complexity in assigning timestamp and duration as we
no longer need to make pseudo-annotations "acquire" and "release".
Finally, this adds tests about above consistently to both scs-zipkin and
scs-zipkin-stream.
