Support multiple marks with the same key

Currently `getMark` returns the High Res mark if the browser supports User Timing instead of the User Timing mark. This is by design (user Timing is just for duration calculations), but results in overwriting values with the same key.

Need a way to have the same key used for multiple marks so we can maintain that User Timing feature if needed.

Related - for more accurate marks when the browser supports both High Res time and User Timing, we should grab the User Timing mark reference to startTime and store that as the high-res mark instead of marking with `window.performance.now()`