Provide an easier way to bootstrap CDI requests in the Netty4 server

Provide a simpler way to integrate CDI into the Netty4 server. The following changes should happen:

• The ResteasyExtension (for CDI) should track resources and providers, and allow them to be loaded dynamically.
• For each incoming request, mimic the CDI behavior of starting a RequestContext for the duration of the call stack.
• Allow the overriding of the request dispatcher, so that other implementations can be used.
• Clean up the handling of CdiInjectorFactory to correctly work when no servlet context is present.