Cache outdated fetched descriptors on directory authorities
When tor starts fetching a descriptor due to a consensus, then replaces its consensus before the descriptor arrives, the descriptor is unrecognised.
Directory caches keep the descriptor in the old descriptor cache, but clients and authorities drop it.
But I think authorities should keep the descriptor, in case it is used in a future vote or consensus.
This is a bug on tor 0.1.1.13-alpha.