invoices: refactor updateHtlc to return new state

With this refactor updateHtlc is renamed to getUpdatedHtlcState and
changed such that it won't change the HTLC's state and resolve time but
instead returns whether the change is needed. This change is part of a
multi-commit refactor to ensure that all changes to the invoice will be
tracked individually.

channeldb/invoice_test.go

@@ -2997,7 +2997,14 @@ func TestUpdateHTLC(t *testing.T) {
 
 func testUpdateHTLC(t *testing.T, test updateHTLCTest) {
 	htlc := test.input.Copy()
-	_, err := updateHtlc(testNow, htlc, test.invState, test.setID)
+	stateChanged, state, err := getUpdatedHtlcState(
+		htlc, test.invState, test.setID,
+	)
+	if stateChanged {
+		htlc.State = state
+		htlc.ResolveTime = testNow
+	}
+
 	require.Equal(t, test.expErr, err)
 	require.Equal(t, test.output, *htlc)
 }