Instead of a boutique message, use a "real" channel_announcement for
private channels (with fake sigs and pubkeys). This makes it far
easier for gossmap to handle local channels.
Backwards compatible update, since we update old stores.
We also fix devtools/dump-gossipstore to know about the tombstone markers.
Since we increment our channel_announce count for local channels now,
the stats in the tests changed too.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
