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/test_utils.go

@@ -844,16 +844,24 @@ func newThreeHopNetwork(t testing.TB, aliceChannel, firstBobChannel,
 	bobDb := firstBobChannel.State().Db
 	carolDb := carolChannel.State().Db
 
+	defaultDelta := uint32(6)
+
 	// Create three peers/servers.
-	aliceServer, err := newMockServer(t, "alice", startingHeight, aliceDb)
+	aliceServer, err := newMockServer(
+		t, "alice", startingHeight, aliceDb, defaultDelta,
+	)
 	if err != nil {
 		t.Fatalf("unable to create alice server: %v", err)
 	}
-	bobServer, err := newMockServer(t, "bob", startingHeight, bobDb)
+	bobServer, err := newMockServer(
+		t, "bob", startingHeight, bobDb, defaultDelta,
+	)
 	if err != nil {
 		t.Fatalf("unable to create bob server: %v", err)
 	}
-	carolServer, err := newMockServer(t, "carol", startingHeight, carolDb)
+	carolServer, err := newMockServer(
+		t, "carol", startingHeight, carolDb, defaultDelta,
+	)
 	if err != nil {
 		t.Fatalf("unable to create carol server: %v", err)
 	}
@@ -883,7 +891,7 @@ func newThreeHopNetwork(t testing.TB, aliceChannel, firstBobChannel,
 	globalPolicy := ForwardingPolicy{
 		MinHTLC:       lnwire.NewMSatFromSatoshis(5),
 		BaseFee:       lnwire.NewMSatFromSatoshis(1),
-		TimeLockDelta: 6,
+		TimeLockDelta: defaultDelta,
 	}
 	obfuscator := NewMockObfuscator()