use get_uptime() consistently
We have this nice function get_uptime() that shields the global variable
stats_n_seconds_working from the rest of the Tor files.
But then we undermine that by saying
extern long stats_n_seconds_working;
in main.h and we just start using that variable directly all over.
There are a few lonely users of get_uptime().
We should move everybody over to using get_uptime, and get rid of the scary extern.
(Also check out how we're writing to the extern variable, in hibernate.c. There's no way that could confuse anybody down the road!)