Pluggable span/trace handlers

[codefromthecrypt]

from https://github.com/openzipkin/zipkin/pull/2072#issuecomment-474802294

I'm inclined to pick this back up with a zipkin2.Span variant of https://github.com/openzipkin/brave/blob/master/brave/src/main/java/brave/handler/FinishedSpanHandler.java#L30

Naming is a funny one, but I like CollectedSpanHandler at the moment. I would add a lot of javadoc mentioning limitations I said including that a "collected span" isn't necessarily a finished one (partial data can be here, such as a span holding nothing but a single tag). Later, you could imagine a CollectedTraceHandler. regardless, these types would be in the zipkin-collector jar.

So, this would result in CollectedSpanHandler and CollectedTraceHandler. As discussed also with @jeqo the latter we'd want an in-memory mechanism to trigger, which could be swapped out in integrations that have partitioned triggers like kafka-streaming, flink and voltdb.

cc @benjefferies @afalko

[codefromthecrypt]

https://github.com/openzipkin/zipkin/pull/2807 single span version