tor-dirmgr: Don't try to mark consensus usable in a read-only store.
Doing this means that any attempt to use a read-only store would crash as soon as it found that the consensus was usable.
It seems that this bug was introduced at some point doing all the dirmgr refactors we did over the past year. Perhaps there should be a test for running with a read-only store.
Fixes #779 (closed)