Add status to the unique key, otherwise unique constraint is too...constraining

When events are cleared with `wp_clear_scheduled_hook()`, they can't be rescheduled until the cleanup job runs.

includes/class-events-store.php

@@ -100,7 +100,7 @@ class Events_Store extends Singleton {
 			`last_modified` datetime NOT NULL,
 
 			PRIMARY KEY (`ID`),
-			UNIQUE KEY `ts_action_instance` (`timestamp`, `action`, `instance`)
+			UNIQUE KEY `ts_action_instance` (`timestamp`, `action`, `instance`, `status`)
 		) ENGINE=InnoDB;\n";
 
 		dbDelta( $schema, true );