Plugins cant get access to their i18n keys during plugin enablement event

After the plug-in (service desk) has been informed that is has been started (PluginEnabledEvent) we do a i18n lookup which fails.

Tested with 6.0-m09 and 6.0-rc1. Only fails during first start with an empty database. eg when you have the database configured BUT nothing in the tables

After that it works. This issue does not exist in JIRA 5.1.

In service desk it leads to custom fields being created with ugly i18n key in them because we cant get access to i18n keys in the plugin started event.

Brad created a test plugin that demonstrates this bug

https://bitbucket.org/atlassian/jira-test-plugins/src/8582d9db83cb/start-order-plugin?at=master

This will print information during the start phases of a plugin. It confirms that during the state where the database has been initialised (and the plugin system inherently restarted) then the i18n key of a plugin are not available to itself.

I suspect we are not getting the "cache clearing" events we think inside the I18nHelper.Factory code.''

The upshot of this bug is that plugins can use i18n keys inside "plugin enabled event" processing to do anything like setup data.