[feature/migrations] Fixing returns of callables and handling data state

Lots of comments and some other miscellaneous fixes.

PHPBB3-9737

phpBB/includes/db/db_tools.php

@@ -349,6 +349,7 @@ class phpbb_db_tools
 	* Setter for {@link $return_statements return_statements}.
 	*
 	* @param bool $return_statements True if SQL should not be executed but returned as strings
+	* @return null
 	*/
 	public function set_return_statements($return_statements)
 	{