timer: change timers_expire() to return a single timer.

timer: change timers_expire() to return a single timer.

The linked list method was problematic, especially if timers delete
other expired timers (eg. the next one in the expired list: the timer_del
will delete it from expired, but that's a bit unexpected).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>