Clean up.
Fix #7459.

Keep track of the measurements of the same name: assign them different ids, but
report them under the same name. All the measurements are now PerfMeasurement
objects.
Fix JSHint error.
Address review comments.
Use this.name for toString() instead of this.id.

This is to preserve the API compatibility.

Also:
    o make the aux function name more descriptive.
Nits and doc.
Merge pull request #9392 from adobe/kai/fix-7459-reentrant-perf-timers

Fix #7459 -- allow parallel timers of the same name.