Code refactoring

- Code refactoring
- Fix a bug that the other player will not become the host of the room at once, when the player disconnect in link play.

> Maybe add many unknown bugs. XD
> The song database `arcsong.db` will not used in the future. You can use a tool in `tool` folder to import old data.

latest version/server/init.py

@@ -1,6 +1,6 @@
 import os
 from shutil import copy, copy2
-from server.sql import Connect
+from core.sql import Connect
 from database.database_initialize import main, ARCAEA_SERVER_VERSION
 from web.system import update_database
 
@@ -91,8 +91,4 @@ def check_before_run(app):
                 app.logger.warning(
                     'Fail to update the file `database/arcaea_database.db`.')
 
-    if not os.path.exists('database/arcsong.db'):
-        app.logger.warning('File `database/arcsong.db` is missing.')
-        f = False
-
     return f