netcheck::Client owns the actor task and when dropped it will abort
the actor task.  Making a struct owning a task Clone means it is easy
to lose track of who should be owning a task like this.  I now believe
each task should have a clear supervisor/owner in charge of it.

This cleans up the multiple-ownership of the netcheck::Client, which
is just a small step into this direction.  Later on, for e.g. #2647,
more supervision will be added.  But small changes are good.