Expire artifacts on artifact share servers
Currently artifact servers are unbounded and will simply grow until system level errors prevent it from functioning correctly, at which point artifacts can no longer be pushed; and manual intervention is required on the server.
This is very to #135, except that the problem of expiring least-recently-used artifacts on the remote server is much more complicated, all because currently if the user only has read-access to the artifact share, it is impossible with the current implementation to record the activity of having downloaded or asked for an artifact.
As an initial step, we might consider deleting the server cache entirely once a quota is reached.
Later, it will be interesting to enhance the protocol with a smart script which is safe to run on the server for unauthenticated users, so that we can record the last time an artifact was used, and then implement a smarter LRU expiry mechanism.