lnwallet: update commitmentKeyRing to account for distinct HTLC keys

In this commit, we update all the key derivation within the state
machine to account for the recent spec change which introduces a
distinct key for usages within all HTLC scripts. This change means that
the commitment payment and delay base points, are only required to be
online in the case that a party is forced to go to chain.

We introduce an additional local tweak to the keyring for the HTLC
tweak. Additionally, two new keys have been added: a local and a remote
HTLC key. Generation of sender/receiver HTLC scripts now use the local
and remote HTLC keys rather than the “payment” key for each party.
Finally, when creating/verifying signatures for second-level HTLC
transactions, we use these the distinct HTLC keys, rather than re-using
the payment keys.

lnwallet/channel_test.go

@@ -225,6 +225,7 @@ func createTestChannels(revocationWindow int) (*LightningChannel, *LightningChan
 	}
 	fundingTxIn := wire.NewTxIn(prevOut, nil, nil)
 
+	// TODO(roasbeef): use distinct keys
 	aliceCfg := channeldb.ChannelConfig{
 		ChannelConstraints: channeldb.ChannelConstraints{
 			DustLimit:        aliceDustLimit,
@@ -238,6 +239,7 @@ func createTestChannels(revocationWindow int) (*LightningChannel, *LightningChan
 		RevocationBasePoint: aliceKeyPub,
 		PaymentBasePoint:    aliceKeyPub,
 		DelayBasePoint:      aliceKeyPub,
+		HtlcBasePoint:       aliceKeyPub,
 	}
 	bobCfg := channeldb.ChannelConfig{
 		ChannelConstraints: channeldb.ChannelConstraints{
@@ -252,6 +254,7 @@ func createTestChannels(revocationWindow int) (*LightningChannel, *LightningChan
 		RevocationBasePoint: bobKeyPub,
 		PaymentBasePoint:    bobKeyPub,
 		DelayBasePoint:      bobKeyPub,
+		HtlcBasePoint:       bobKeyPub,
 	}
 
 	bobRoot := DeriveRevocationRoot(bobKeyPriv, testHdSeed, aliceKeyPub)