Per-deployment web thread pools

It would be nice if you could limit the number of worker threads being used by an application, and one way of achieving that would be to add a per-deployment thread pool for handing web requests.

This can be done currently by configuring one <connector> per application and giving each one it's own thread pool.

Undertow supports (from cursory inspection) DeploymentInfo.setExecutor/setAsyncExecutor() which configured the Executor to use for requests. The subsystem could read a thread pool configuration from jboss-web.xml and pass the resulting executor into those two methods to make setting this up easier and not require a load balancer.