Instead of hardcoding the minimum fraction of possible paths to 0.6, we
take it from the user, and failing that from the consensus, and
failing that we fall back to 0.6.
Previously we did this based on the fraction of descriptors we
had. But really, we should be going based on what fraction of paths
we're able to build based on weighted bandwidth, since otherwise a
directory guard or two could make us behave quite oddly.
Implementation for feature 5956