Stem now provides the ability to create and sign descriptors, so
there's no reason to use leekspin anymore.  Stem is faster at creating
descriptors and we're already heavily depending on stem anyway.

This patch improves the preliminary work that Isis Lovecruft and
Damian Johnson already did.  Some code in this patch was taken from
leekspin.

This fixes bug 22755: <https://bugs.torproject.org/22755>