graph/db+sqldb: implement UpdateEdgePolicy

In this commit, the various SQL queries are defined that we will need in
order to implement the SQLStore UpdateEdgePolicy method. Channel
policies can be "replaced" and so we use the upsert pattern for them
with the rule that any new channel policy must have a timestamp greater
than the previous one we persisted.

As is done for the KVStore implementation of the method, we use the
batch scheduler for this method.

graph/db/graph_test.go

@@ -4038,7 +4038,7 @@ func TestBatchedAddChannelEdge(t *testing.T) {
 func TestBatchedUpdateEdgePolicy(t *testing.T) {
 	t.Parallel()
 
-	graph := MakeTestGraph(t)
+	graph := MakeTestGraphNew(t)
 
 	// We'd like to test the update of edges inserted into the database, so
 	// we create two vertexes to connect.