Implement trait-based ResourceThreads and clean up related naming issues

Changes include:

- Introduce an IpcSend trait to abstract over a collection of IpcSenders
- Implement ResourceThreads collection to abstract the resource-related
  sub threads across the component
- Rename original ResourceThread and ControlMsg into an unifed CoreResource__
  to accommodate above changes and avoid confusions

components/script/dom/dedicatedworkerglobalscope.rs

@@ -224,7 +224,7 @@ impl DedicatedWorkerGlobalScope {
             let _stack_roots_tls = StackRootTLS::new(&roots);
 
             let (url, source) = match load_whole_resource(LoadContext::Script,
-                                                          &init.resource_thread,
+                                                          &init.core_resource_thread,
                                                           worker_url,
                                                           None) {
                 Err(_) => {