Expose URL as the title of toplevel async slices

Exposing the url param as the title of the toplevel slice improves readability of netlog events. This change adds an async_slice_track that processes each toplevel async slice to find the url param in itself or its descendants if there is a url param, and replaces the title with this url param.
Well, you're effectively modifying the model here by creating new model objects on the fly. I'm just concerned these won't be hooked into things correctly or will be missing when we subclass the model objects.

The other option is, can we change get title() on the model object to 'do the right thing' and return the title or URL? That way we aren't mutating the model, it just does the right thing.

Not sure what you'd need to do to flag the fact that the title is to be grabbed from params?