hs: in NetDir, wrap HsDirs in an Arc<>.

Once the HsDirs has been constructed for a given NetDir, it will never change. But while we are filling in a NetDir, we sometimes need to Clone the NetDir in order to add new information to the 'official' copy while other instances of the NetDir are in use. (See use of Arc::make_mut in dirmgr.)

To this end, we try to keep the immutable data in NetDir behind an Arc, so that Clone can be as shallow a copy as possible. Therefore, it makes sens to stick the HsDirs in an Arc.

@Diziet if you agree with the reasoning here I can do the patch.