Require live consensus to compute responsible HSDirs.

Here is how this changes the HSv3 client-side and service-side:

For service side we already required live consensus to upload descriptors (see
9e900d1d) so we should never get there without
a live consensus.

For the client-side we now require a live consensus to attempt to connect to
HS.  While this changes the client behavior in principle, it doesn't really
change it, because we always required live consensus to set HSDir indices, so
before this patch a client with no live consensus would try to compute
responsible HSDirs without any HSDir indices and bug out. This makes the client
behavior more consistent, by requiring a live consensus (and hence a
semi-synced clock) for the client to connect to an HS entirely.

The alternative would have been to allow setting HSDir indices with a non-live
consensus, but this would cause the various problems outlined by commit
b89d2fa1.