gossipd: rewrite to do the handshake internally.

Now the flow is much simpler from a lightningd POV:

1. If we want to connect to a peer, just send gossipd `gossipctl_reach_peer`.
2. Every new peer, gossipd hands up to lightningd, with global/local features
   and the peer fd and a gossip fd using `gossip_peer_connected`
3. If lightningd doesn't want it, it just hands the peerfd and global/local
   features back to gossipd using `gossipctl_handle_peer`
4. If a peer sends a non-gossip msg (eg `open_channel`) the gossipd sends
   it up using `gossip_peer_nongossip`.
5. If lightningd wants to fund a channel, it simply calls `release_channel`.

Notes:
* There's no more "unique_id": we use the peer id.
* For the moment, we don't ask gossipd when we're told to list peers, so
  connected peers without a channel don't appear in the JSON getpeers API.
* We add a `gossipctl_peer_addrhint` for the moment, so you can connect to
  a specific ip/port, but using other sources is a TODO.
* We now (correctly) only give up on reaching a peer after we exchange init
  messages, which changes the test_disconnect case.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/netaddr.h

@@ -1,3 +1,4 @@
+/* FIXME: We should deprecate this in favor of BOLT7 address descriptor */
 #ifndef LIGHTNING_LIGHTNINGD_NETADDR_H
 #define LIGHTNING_LIGHTNINGD_NETADDR_H
 #include "config.h"