shared_random: Improve fallback for client no-live-consensus case.

In this case, when we're looking for the voting interval, we should
try looking at the _latest_ consensus if we have one.  When we're
looking for the start of the current voting period, we can use our
existing fallback logic without complaint, since the voting interval
code will already have given us a reasonable voting interval, and we
want to have a round starting time based on the current time.