fix(instruments): Set the returned closure's name to the handler name

When timing an async function we return a new closure that starts and
stops the timer to measure the time the async function was running. This
change ensures we don't clobber the timed function's name when we use
this timed wrapper.
Merge pull request #15 from racker/jirwin/set-name-of-timed-async-function

fix(instruments): Set the returned closure's name to the handler name