Tweak consensus method 17 based on arma's comments

Instead of capping whenever a router has fewer than 3 measurements,
we cap whenever a router has fewer than 3 measurements *AND* there
are at least 3 authorities publishing measured bandwidths.

We also generate bandwidth lines with a new "Unmeasured=1" flag,
meaning that we didn't have enough observations for a node to use
measured bandwidth values in the authority's input, whether we capped
it or not.

src/or/routerparse.c

@@ -1985,6 +1985,7 @@ routerstatus_parse_entry_from_string(memarea_t *area,
           goto err;
         }
         rs->has_measured_bw = 1;
+        vote->has_measured_bws = 1;
       }
     }
   }