tevent: optimize adding new timer events

tevent: optimize adding new timer events

There're two cases:

1. Adding a timer with a zero timestamp.
   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.

2. Adding a timer with a real timestamp.
   As this 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.

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