discovery: modify message validation to match BOLT-0007
discovery: modify message validation to match BOLT-0007

This commit implements the simplification within the latest version of
BOLT-0007. With this change, we simply sign the message hash directly
with the bitcoin keys, rather than signing the node keys themselves.

LightningNetwork/lnd,ddustin/lnd,LightningNetwork/lnd,aakselrod/lnd,ddustin/lnd,aakselrod/lnd