lnwallet: only forward freshly locked in HTLC's in ReceiveRevocation

This commit fixes a nasty bug that has been lingering within lnd, and
has been noticed due to the added retransmission logic. Before this
commit, upon a restart, if we had an active HTLC and received a new
commitment update, then we would re-forward ALL active HTLC’s. This
could at times lead to a nasty cycle:
  * We re-forward an HTLC already processed.
  * We then notice that the time-lock is out of date (retransmitted
HTLC), so we go to fail it.
  * This is detected as a replay attack, so we send an
UpdateMalformedHTLC
  * This second failure ends up creating a nil entry in the log,
leading to a panic.
  * Remote party disconnects.
  * Upon reconnect we send again as we need to retransmit the changes,
this goes on forever.

In order to fix this, we now ensure that we only forward HTLC’s that
have been newly locked in at this next state. With this, we now avoid
the loop described above, and also ensure that we don’t accidentally
attempt an HTLC replay attack on our selves.

Fixes #528.

Fixes #545.

lnwallet/channel.go

@@ -3905,18 +3905,27 @@ func (lc *LightningChannel) ReceiveRevocation(revMsg *lnwire.RevokeAndAck) ([]*P
 			continue
 		}
 
+		// We'll only forward any new HTLC additions iff, it's "freshly
+		// locked in". Meaning that the HTLC was only *just* considered
+		// locked-in at this new state. By doing this we ensure that we
+		// don't re-forward any already processed HTLC's after a
+		// restart.
 		if htlc.EntryType == Add &&
-			remoteChainTail >= htlc.addCommitHeightRemote &&
+			remoteChainTail == htlc.addCommitHeightRemote &&
 			localChainTail >= htlc.addCommitHeightLocal {
 
 			htlc.isForwarded = true
 			htlcsToForward = append(htlcsToForward, htlc)
-		} else if htlc.EntryType != Add &&
+			continue
+		}
+
+		if htlc.EntryType != Add &&
 			remoteChainTail >= htlc.removeCommitHeightRemote &&
 			localChainTail >= htlc.removeCommitHeightLocal {
 
 			htlc.isForwarded = true
 			htlcsToForward = append(htlcsToForward, htlc)
+			continue
 		}
 	}