We can have more than one; eg we might offer both bech32 and a p2sh
address, and in future we might offer v1 segwit, etc.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
'rhash' is the old terminology, but 'payment_preimage' and
'payment_hash' were decided on for the BOLTs, so we should fix that here.
We still use rhash internally, but that's much easier to fix.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>