lnwallet: properly handle HTLC settles in channel state-machine

We no longer track HTLC’s by their r-hash within the log into the
index, as we may have multiple HTLC’s that can be redeemed by the same
pre-image. Instead we now use a separate index which is keyed by a
log-index.

Additionally, the SettleHTLC method now also returns the index of the
HTLC being settled which allows the remote party to quickly locate the
HTLC within their log.

This commit also introduces a few trace/debug log messages which will
likely be pruned in the near future

lnwallet/channel_test.go

@@ -329,13 +329,10 @@ func TestSimpleAddSettleWorkflow(t *testing.T) {
 	// HTLC once he learns of the preimage.
 	var preimage [32]byte
 	copy(preimage[:], paymentPreimage)
-	wireSettleMsg := &lnwire.HTLCSettleRequest{
-		RedemptionProofs: [][32]byte{preimage},
-	}
-	if err := bobChannel.SettleHTLC(wireSettleMsg, false); err != nil {
+	if _, err := bobChannel.SettleHTLC(preimage, false); err != nil {
 		t.Fatalf("bob unable to settle inbound htlc: %v", err)
 	}
-	if err := aliceChannel.SettleHTLC(wireSettleMsg, true); err != nil {
+	if _, err := aliceChannel.SettleHTLC(preimage, true); err != nil {
 		t.Fatalf("alice unable to accept settle of outbound htlc: %v", err)
 	}
 	bobSig2, aliceIndex2, err := bobChannel.SignNextCommitment()