htlcswitch: Assign each pending payment a unique ID.

This simplifies the pending payment handling code because it allows it
be handled in nearly the same way as forwarded HTLCs by treating an
empty channel ID as local dispatch.

htlcswitch/link_test.go

@@ -1448,7 +1448,7 @@ func newSingleLinkTestHarness(chanAmt btcutil.Amount) (ChannelLink, func(), erro
 	aliceCfg := ChannelLinkConfig{
 		FwrdingPolicy:     globalPolicy,
 		Peer:              &alicePeer,
-		Switch:            nil,
+		Switch:            New(Config{}),
 		DecodeHopIterator: decoder.DecodeHopIterator,
 		DecodeOnionObfuscator: func(io.Reader) (ErrorEncrypter, lnwire.FailCode) {
 			return obfuscator, lnwire.CodeNone