zipkin: fix span duration stats in zipkin realtime
 - fix bugs in span duration stats
 - add tests for monoid

RB_ID=378081
moved Kafka receiver to ThriftSpan

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=391305
Updating install info for zipkin web

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=391323
fix zipkin-example
sync oss