Now we've make it only on existing channels, and not have to call
listdatastore every time, that means we can safely turn it on by
default.
Changelog-Added: Protocol: we now offer peer storage to any peers who create a channel.
Changelog-Deprecated: Config: `--experimental-peer-storage` (it's now the default).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>