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.