MockTracer.close() clears finishedSpans

[t-8ch]

I am trying to use MockTracer for unit testing.
For this I execute my application and then validate the contents of MockTracer.finishedSpans().
For 0.32 I modified my application to take advantage of the new Tracer.close() method to properly free Tracer resources.
This broke the tests because on closing the MockTracer its finishedSpans are reset.

I would argue that

• Closing the tracer should not reset the finishedSpans
• The internal isClosed member should be accessible to users of MockTracer to ensure that the tracer has been properly closed.