B-58460 Request tracer perf tweak

I've made a few changes here that weren't covered in the story description. First, I choose to remove the code that capture the initialization time for the RequestTracer object. As far as I can tell, there is no need for it. Second, I no longer leave it to the user to keep track of the start time (although it is still provided in the return from traceEnter()). Third, I messed with the actual timing code a bit. I noticed in my testing that the first set of calls to traceEnter() and traceExit(...) would return the delay I was expecting, but subsequent calls would merely return the difference. 

I'm not sure if the last change was an issue since we were only using RequestTracer in one spot (in the PowerFilterChain) and in a very specific way. Still, my proposed changes should allow for more confidence in the correctness and consistency of this class.