OAUTH TokenExpiry never starts in Bamboo 3.0

When upgrading to the new Atlassian Plaform, we've changed the expiry scheduler to be triggered by Bamboo start instead of plugin system startup event.
This is a good idea, but the plugin was not wired properly to receive the startup event. As the result, expiry never runs on Bamboo 3.0.
Priority major since the exposure is limited, but this has to be fixed fast.