We currently create a peer struct, then complete handshake to find out
who it is. This means we have a half-formed peer, and worse: if it's
a reconnect we get two peers the same.
Add an explicit 'struct connection' for the handshake phase, and
construct a 'struct peer' once that's done.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
