sched: keep track of thread's runtime

This will be used later to determine for how long have a thread been running.
It can easily be updated right before we call ran_for(), reusing its interval
parameter.

Fixes #135

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>