tevent: optimize adding new zero timer events

Such events were used before we had immediate events.
It's likely that there're a lot of this events
and we need to add new ones in fifo order.

The tricky part is that tevent_common_add_timer()
should not use the optimization as it's used
by broken Samba versions, which don't use
tevent_common_loop_timer_delay() in source3/lib/events.c.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>