Delay listener initialization so that it happens after execution of servlet container initializers

Listeners are currently initialized eagerly and even before any servlet container initializers were created/executed. This prevents Weld integration and subsequently makes it impossible to perform injection into listeners because Weld attempts to register EnhancedListener (implementing ServletContainerInitializer) in order to bootstrap and only after that it is able to serve Undertow bean instances of listeners.

 
This has been hidden bug for a long time as Weld test verifying this was using very old Undertow version where this worked.

I've gone over the details with flaviarnn via Zulip and I'll send a PR shortly.