[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/install/schemas/mysql_41_schema.sql

@@ -195,6 +195,7 @@ CREATE TABLE phpbb_drafts (
 CREATE TABLE phpbb_ext (
 	ext_name varchar(255) DEFAULT '' NOT NULL,
 	ext_active tinyint(1) UNSIGNED DEFAULT '0' NOT NULL,
+	ext_state text NOT NULL,
 	UNIQUE ext_name (ext_name)
 ) CHARACTER SET `utf8` COLLATE `utf8_bin`;