idea: add response headers with high-res timing information in test server replies

If the test server could be asked to add response headers with high-res timing information about when it received a request, and when it started replying (or just the delta between those two), and Nighthawk's client could opt-in to track those in its histograms, that might give end users a means to verify that the latencies from the test server's point of view line up with what the client observed. If those diverge, that is interesting information.