narrow down redis info scope and switch to "used_memory_dataset"

There is no need to fetch full server info, we only read from the memory
section. Also, "used_memory" might be misleading as this represents the
total memory allocation of the server instance. It may be confusing if
there are several megabyes allocated after flushing the cache, so
"used_memory_dataset" which subtracts the internal overhead is closer to
what a user might expect to see here.

inc/class-cachify-redis.php

@@ -142,19 +142,19 @@ public static function get_stats() {
 		}
 
 		/* Info */
-		$data = self::$_redis->info();
+		$data = self::$_redis->info( 'MEMORY' );
 
 		/* No stats? */
 		if ( empty( $data ) ) {
 			return null;
 		}
 
 		/* Empty */
-		if ( empty( $data['used_memory'] ) ) {
+		if ( empty( $data['used_memory_dataset'] ) ) {
 			return null;
 		}
 
-		return $data['used_memory'];
+		return $data['used_memory_dataset'];
 	}
 
 	/**