Adds support for "finished span handler"

[jcchavezs]

There are some cases where one might need to enable/disable tracing for certain children spans. For example, if I will write a bunch of files in a zip stream, I want to trace the call to S3 to list the files but not every single call to S3 to retrieve the content of the file. In such case I want to make a decision on what to record and what not based on rules in the spans.

For implementation reference we could have a look at brave: https://github.com/openzipkin/brave/blob/master/brave/src/main/java/brave/handler/FinishedSpanHandler.java

Ping @basvanbeek @adriancole

[basvanbeek]

FinishedSpanHandler would also allow for a solution to #58