db: enforce that bindings be done in order.

This is almost always true already; fix up the few non-standard ones.

This is enforced with an assert, and I ran the entire test suite to
double-check.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

db/common.h

@@ -104,6 +104,9 @@ struct db_stmt {
 	/* Our entry in the list of pending statements. */
 	struct list_node list;
 
+	/* Bind counter */
+	int bind_pos;
+
 	/* Database we are querying */
 	struct db *db;