DirMgr: Detect that directory state is ready if it begins with all MDs
Previously, we'd only call PendingNetDir::upgrade_if_necesessary when adding a microdescriptor. But if it began already having all of its descriptors (because we found them in the cache), we wouldn't actually upgrade it to a PendingNetDir::Yielding, which would make it unusable, and would make us schedule its reset time too far in the future.
Fixes #802 (closed).