Change relay_msg_t to _not_ hold a copy of the message.

Change relay_msg_t to _not_ hold a copy of the message.

Previously we had to memdup every time we parsed a relay_msg_t;
but that's unnecessary, since (most) every time we use it, we have
a longer-lived cell object.

This _did_ require some hacking in relay_msg_copy, but I think the
gain in simplicity is worth it.