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!)