tox_friend_send_lossless_packet return true, but send message failed

[randoms]

1. Start two tox node, tox1 and tox2. Tox1 and tox2 are connected.
2. Kill tox2 and call tox_friend_send_lossless_packet in tox1 immediately, tox_friend_send_lossless_packet will return true, but in fact the message was failed to send.

[iphydf]

Toxcore has no persistence. Lossless packets are only mildly guaranteed lossless. We need a better approach to actual lossless transmissions and toxcore is probably the wrong place to do this. I think lossless packets inside toxcore should go away entirely and be replaced by a layer on top of lossy packets.