Replaced the old timing functions by something more useful.

Replaced the old timing functions by something more useful.

get_timer() using global state was making it unusable in many cases,
especially in library routines. I decided to make the callers specify
an explicit context (a timer) instead and re-used the idea of millisecond
timestamps from mainloop.c.

I've moved timestamp_t to config.h, introduced global get_timestamp()
and replaced the timer functions by ones with an explicit timestamp_t *
argument.