Fix premature trace reset

If a child span finished (such as a client request happening inside of a
server request trace) the whole trace was being cleared. Thus, when the
parent span finally finished, all of the previous annotations were lost.

Instead, track annotations only by spanId and don't reset the whole
trace when a span finishes.

Test included which demonstrates the failure.
Merge branch 'fix-premature-trace-reset' of github.com:wadey/node-tryfer into wadley_fix-premature-trace-reset
Keep track of span IDs in the context of trace IDs, since span IDs may not be globally unique across all trace IDs
Merge pull request #13 from racker/wadley_fix-premature-trace-reset

Alteration to wadley:fix-premature-trace-reset