Upload descriptors more often when recent desc is unlisted

Right now we only force a new descriptor upload every 18 hours.
This can make servers become unlisted if they upload a descriptor at
time T which the authorities reject as being "too similar" to one
they uploaded before. Nothing will actually make the server upload a
new descriptor later on, until another 18 hours have passed.

This patch changes the upload behavior so that the 18 hour interval
applies only when we're listed in a live consensus with a descriptor
published within the last 18 hours.  Otherwise--if we're not listed
in the live consensus, or if we're listed with a publication time
over 18 hours in the past--we upload a new descriptor every 90
minutes.

This is an attempted bugfix for #3327.  If we merge it, it should
obsolete #535.