lnwallet: pack paymentDescriptor add/remove heights into Duals

The purpose of this commit is to begin the process of packing
symmetric fields into the newly introduced Dual structure. The
reason for this is that the Dual structure has a handy indexing
method where we can supply a ChannelParty and get back a value.
This will cut down on the amount of branching code in the main
lines of the codebase logic, making it easier to follow what is
going on.

lnwallet/aux_signer.go

@@ -109,10 +109,10 @@ func newAuxHtlcDescriptor(p *paymentDescriptor) AuxHtlcDescriptor {
 		ParentIndex:              p.ParentIndex,
 		EntryType:                p.EntryType,
 		CustomRecords:            p.CustomRecords.Copy(),
-		addCommitHeightRemote:    p.addCommitHeightRemote,
-		addCommitHeightLocal:     p.addCommitHeightLocal,
-		removeCommitHeightRemote: p.removeCommitHeightRemote,
-		removeCommitHeightLocal:  p.removeCommitHeightLocal,
+		addCommitHeightRemote:    p.addCommitHeights.Remote,
+		addCommitHeightLocal:     p.addCommitHeights.Local,
+		removeCommitHeightRemote: p.removeCommitHeights.Remote,
+		removeCommitHeightLocal:  p.removeCommitHeights.Local,
 	}
 }