Migration Guide: missing content on migrating the "threads" subsystem

The threads subsystem was used to manage threadpools in EAP 6 for a lot of subsystems. See the subtasks of WFLY-462 for the list. We should have a dedicated chapter under Server Config Changes to describe migrating from the threads subsystem to the new threadpool management scheme. I think that, generally, all the subsystems that used the threads subsystem now have their own facility to manage their own threadpools.

In JBEAP-2789, we added the related content for the Infinispan subsystem. We need to add content for the remaining ones. I think that it would be enough to describe the change in general (something like: "the threads subsystem from EAP 6 was removed; in EAP 7, each subsystem is supposed to manage threadpools on their own") and then add specifics for each subsystem.

I think the most challenging ones will be messaging (replaced by messaging-activemq, but that should have a lot of similarities to the old messaging subsystem) and web (replaced by undertow, a completely new thing that doesn't have any similarity to web).