Maintains a list of directories used by workers connected to this server. Only these directories are fetchable with dead-worker-fetch. Each directory indexes a random has that is the name of a file that the worker should generate. This way a worker can prove that it can write to the directory it claims is the worker directory. This closes a potential security issue where the worker could make the server read any file.
check whether a requested directory is a worker directory. return True if the directory 'dir' exists, False if it doesn't exist, and raise an exception if the access is denied (for example, when the random file wasn't created).