Attach App Engine Runtime source artifacts async

Currently the sources for the App Engine runtime jars (servlet, jsp) are resolved synchronously. This could be changed to make it async.

Implementation details: the `project` parameter of `com.google.cloud.tools.eclipse.appengine.localserver.ServletClasspathProvider.doResolveClasspathContainer(IProject, IRuntime)` is not passed on to `ILibraryRepositoryService` leading to synchronous resolution. However simply passing it is not sufficient as the logic in the background job needs to be changed to be able to correctly find which classpath entry needs to be modified.