Removed a bunch of useless space from various structs.
pack_nodes() and unpack_nodes() are now used to transmit lists of
nodes for the DHT and onion parts. They provide a way to transmit
both ipv6 and ipv4 nodes in the same packet in a portable way that
takes as little space as possible without compression.
Using pack_nodes, merged the send nodes and send nodes ipv6 packets
into one packet greatly reducing the risk of amplification attacks.
