lnwallet: update channel state machine to be aware of tweakless commits

In this commit, we update the channel state machine to be aware of
tweakless commits. In several areas, we'll now check the channel's type
to see if it's `SingleFunderTweakless`. If so, then we'll opt to use the
remote party's non-delay based point directly in the script, skipping
any additional cryptographic operations. Along the way we move the
`validateCommitmentSanity` method to be defined _before_ it's used as is
cutomary within the codebase.

Notably, within the `NewUnilateralCloseSummary` method, we'll now _blank
out_ the `SingleTweak` value if the commitment is tweakless. This
indicates to callers the witness type they should map to, as the value
isn't needed at all any longer when sweeping a non-delay output.

We also update the signing+verification tests to also test that we're
able to properly generate a valid witness for the new tweakless
commitment format.

channeldb/channel.go

@@ -869,6 +869,14 @@ func (c *OpenChannel) ChanSyncMsg() (*lnwire.ChannelReestablish, error) {
 	// allowing us to sweep our funds.
 	if c.hasChanStatus(ChanStatusRestored) {
 		currentCommitSecret[0] ^= 1
+
+		// If this is a tweakless channel, then we'll purposefully send
+		// a next local height taht's invalid to trigger a force close
+		// on their end. We do this as tweakless channels don't require
+		// that the commitment point is valid, only that it's present.
+		if c.ChanType.IsTweakless() {
+			nextLocalCommitHeight = 0
+		}
 	}
 
 	return &lnwire.ChannelReestablish{