improve reporting of directory bootstrap progress
Right now, bootstrap events related to directory download progress are misleading, in that they can indicate that we've made progress that we have not actually made yet.
requesting_status when we've unblocked directory download progress by completing a directory circuit, not when we've actually transmitted the request to a directory server.
Similarly, we report
requesting_descriptors when we've noticed that we need to do so, rather than when we've transmitted the request to a directory server.