[feature/extension-manager] Use an incremental process for enable and purge

The enable or purge operation of an extension could take a long time if an
expensive operation needs to be executed on a large set of data. To allow
this to succeed from a web interface with max_execution_time set in the
webserver's php configuration, subsequent requests must continue the
operation started earlier. So individual enable and purge implementations
must be able to spread their work across multiple steps.

PHPBB3-10323

phpBB/includes/extension/base.php

@@ -16,20 +16,38 @@ if (!defined('IN_PHPBB'))
 }
 
 /**
+* A base class for extensions without custom enable/disbale/purge code.
 *
 * @package extension
 */
 class phpbb_extension_base implements phpbb_extension_interface
 {
-	public function enable()
+	/**
+	* Single enable step that does nothing
+	*
+	* @return false Indicates no further steps are required
+	*/
+	public function enable_step($old_state)
 	{
+		return false;
 	}
 
+	/**
+	* Empty disable method
+	*
+	* @return null
+	*/
 	public function disable()
 	{
 	}
 
-	public function purge()
+	/**
+	* Single purge step that does nothing
+	*
+	* @return false Indicates no further steps are required
+	*/
+	public function purge_step($old_state)
 	{
+		return false;
 	}
 }