Don't look at any routerstatus_t when the networkstatus is inconsistent

For a brief moment in networkstatus_set_current_consensus(), the old
consensus has been freed, but the node_t objects still have dead
pointers to the routerstatus_t objects within it.  During that
interval, we absolutely must not do anything that would cause Tor to
look at those dangling pointers.

Unfortunately, calling the (badly labeled!) current_consensus macro
or anything else that calls into we_use_microdescriptors_for_circuits(),
can make us look at the nodelist.

The fix is to make sure we identify the main consensus flavor
_outside_ the danger zone, and to make the danger zone much much
smaller.

Fixes bug 20103.  This bug has been implicitly present for AGES; we
just got lucky for a very long time.  It became a crash bug in
0.2.8.2-alpha when we merged 35bbf2e4 to make
find_dl_schedule start looking at the consensus, and 4460feaf
which made node_get_all_orports less (accidentally) tolerant of
nodes with a valid ri pointer but dangling rs pointer.