Don't set HSDir index if we don't have a live consensus.

We also had to alter the SRV functions to take a consensus as optional
input, since we might be setting our HSDir index using a consensus that
is currently being processed and won't be returned by the
networkstatus_get_live_consensus() function.

This change has two results:

a) It makes sure we are using a fresh consensus with the right SRV value
   when we are calculating the HSDir hash ring.

b) It ensures that we will not use the sr_get_current/previous()
   functions when we don't have a consensus which would have falsely
   triggered the disaster SRV logic.