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
Initial mongodb commit
Redo database schema to use arrays as opposed to dynamically generated keys

Dynamically generated keys would be hard to index, and are probably ill-advised
makeAsync calls to a closed Asyncifier will get a failed future, as opposed to a future that will never return
Implement tracesExist
Implement getSpanNames
Implement getAllServiceNames
Implement getTimeToLife
Implement setTimeToLife
Implement getSpansByTraceId and friends
Test MongoDBSpanStore
Add timestamps to traces and implement getTraceIdsByName
Implement getTracesDuration
Implement getTraceIdsByAnnotation
Work properly for the case when there are multiple span entries in the set for the same id
Remove use of $min and $max, since they are unsupported in mongo 2.4