Fix queue handling in meshlink_send().

We need to copy the data from the application *here*. To avoid multiple copies, prepare the
whole vpn_packet_t in meshlink_send(), so meshlink_send_from_queue() only has to dequeue
and call route(). We also don't need to take the mesh_mutex, since the only information we
use is the name of ourself and the destination node, which is guaranteed to be stable.