archiver: Improve 'unknown sha1' and 'force copy' policies

The 'unknown sha1 path' cannot happen in the default archiver since it
reads from the archive db (so the fallback code is not necessary in the
worker).

To the contrary, since 'archiver to backend' reads from stdin (for now),
we could have unregistered sha1s from that source.

This commit makes the director deal with that before sending sha1 to
workers.

It's also the director's job to set the state to 'missing' when the
force_copy is true before sending sha1 to worker.