Add the ability to reuse an existing `XnioWorker` in `Undertow`.
Add the ability to reuse an existing `XnioWorker` in `Undertow`.

This allows continued use of `Undertow` and its `Builder` even when
you want to share workers.

This is particularly beneficial for usage in higher-level frameworks
that use `Undertow` and its `Builder` rather than manually assembling
a server.

For example, spring-boot uses `Undertow` rather than manually assembling
a server.  So, you will now be able to reuse an `XnioWorker` when using
untertow with spring-boot (by configuring a `org.springframework.boot.context.embedded.undertow.UndertowBuilderCustomizer`).

darranl/undertow,soul2zimate/undertow,undertow-io/undertow,jamezp/undertow,baranowb/undertow,darranl/undertow,aldaris/undertow,Karm/undertow,jamezp/undertow,n1hility/undertow,pferraro/undertow,Karm/undertow,aldaris/undertow,rhusar/undertow,stuartwdouglas/undertow,undertow-io/undertow,popstr/undertow,soul2zimate/undertow,wildfly-security-incubator/undertow,marschall/undertow,rhusar/undertow,rhusar/undertow,ctomc/undertow,baranowb/undertow,ctomc/undertow,jstourac/undertow,jstourac/undertow,jamezp/undertow,msfm/undertow,marschall/undertow,golovnin/undertow,darranl/undertow,pedroigor/undertow,Karm/undertow,n1hility/undertow,aradchykov/undertow,aldaris/undertow,wildfly-security-incubator/undertow,golovnin/undertow,jstourac/undertow,pedroigor/undertow,wildfly-security-incubator/undertow,soul2zimate/undertow,ctomc/undertow,popstr/undertow,pferraro/undertow,msfm/undertow,marschall/undertow,aradchykov/undertow,baranowb/undertow,aradchykov/undertow,msfm/undertow,golovnin/undertow,undertow-io/undertow,stuartwdouglas/undertow,pedroigor/undertow,pferraro/undertow,popstr/undertow,stuartwdouglas/undertow,n1hility/undertow