tevent: optimize adding new timer events

tevent: optimize adding new timer events

As new timestamps typically get higher:-)
it's better to traverse the existing list from
the tail.

This is not completely optimal, but it should be better
than before.

A second optimization could be done for zero timestamps,
we would just remember the last_zero_timer,
but that would change the internal ABI.
Normally thatshould not be a poblem, but the Samba's
source3/lib/events.c abuses tevent_internal.h
from the current source tree, even if an external tevent.h
is used. The other problem is that it makes use of
tevent_common_add_timer() without using
tevent_common_loop_timer_delay().

Signed-off-by: Stefan Metzmacher <metze@samba.org>