Only authorities should automatically download v2 networkstatus documents

Clients and relays haven't used them since early 0.2.0.x.  The only
remaining use by authorities learning about new relays ahead of scedule;
see proposal 147 for what we intend to do about that.

We're leaving in an option (FetchV2Networkstatus) to manually fetch v2
networkstatuses, because apparently dnsel and maybe bwauth want them.

This fixes bug 3022.