In 0.2.3.18-rc, we started warning on this case while building a
list of missing microdescriptor digests. That turned out to spam
the logs; instead let's warn at parse time.
Partial fix for bug 6404.
The spec requires that every router in a microdesc consensus have an
m line; we weren't obeying that spec.
This creates a new consensus method (13) to allow voting to continue
to work right. Partial fix for bug 6404; fix on 0.2.2.6-alpha.