Trickle: remove vtimers before (re)scheduling it

[OlegHahm]

Otherwise it might cause an endless loop in queue.c due to multiple nodes of the same instance (e.g. dao_timer).

(Would look like:

$5 = (struct queue_node_t *) 0x809b1e0 <dao_timer>
(gdb) print node->next->next->next->next
$6 = (struct queue_node_t *) 0x8083dfc <con_buf+7644>
(gdb) print node->next->next->next->next->next
$7 = (struct queue_node_t *) 0x809b1e0 <dao_timer>
(gdb) print node->next->next->next->next->next->next
$8 = (struct queue_node_t *) 0x8083dfc <con_buf+7644>
(gdb) print node->next->next->next->next->next->next->next
$9 = (struct queue_node_t *) 0x809b1e0 <dao_timer>
(gdb) print node->next->next->next->next->next->next->next->next
$10 = (struct queue_node_t *) 0x8083dfc <con_buf+7644>
(gdb) print node->next->next->next->next->next->next->next->next->next
$11 = (struct queue_node_t *) 0x809b1e0 <dao_timer>

)

[LudwigKnuepfer]

Apparently helps, ACK.