htlcswitch: obtain the min final CLTV delta directly from the InvoiceDatabase

In this commit, we modify the existing logic that would attempt to read
the min CLTV information from the invoice directly. With this route, we
avoid any sort of DB index modifications, as this information is already
stored within the payment request, which is already available to the
outside callers. By modifying the InvoiceDatabase interface, we avoid
having to make the switch aware of what the "primary" chain is.

htlcswitch/link.go

@@ -2167,7 +2167,9 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 			// we attempt to see if we have an invoice locally
 			// which'll allow us to settle this htlc.
 			invoiceHash := chainhash.Hash(pd.RHash)
-			invoice, err := l.cfg.Registry.LookupInvoice(invoiceHash)
+			invoice, minCltvDelta, err := l.cfg.Registry.LookupInvoice(
+				invoiceHash,
+			)
 			if err != nil {
 				log.Errorf("unable to query invoice registry: "+
 					" %v", err)
@@ -2258,7 +2260,6 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 
 			// We'll also ensure that our time-lock value has been
 			// computed correctly.
-			minCltvDelta := uint32(invoice.Terms.FinalCltvDelta)
 			expectedHeight := heightNow + minCltvDelta
 			switch {