Fixed timer lifespan in 2013 advent cal async example

[nrdvana]

This example appeared to work, but actually most requests were
having their timer cut short by the next request, because there
was only one global variable, and the program needed to hold
refs to multiple concurrent timers.

Solved in the simplest possible way by moving the timer ref
inside the timer callback to create a circular reference, though
aa better way would be to hold the timer ref in a global pool
and not form the circular ref in the first place. But, that's
more code...