Implement a "tape drive" kind of Store

[pkgw]

To support tracking of tape-drive backups of files on-site, we should support a special kind of Store that is not a regular SSH-able host. Instead it is just a symbolic entry representing a set of tapes on which we've written data.

In practice this might be more of a "write-only" store than a "tape-drive" store: the things that make this store different than a pot are not likely to be specific to tapes in any way.

CC @plaplant

[pkgw]

Note that when the Librarian thinks about how many "instances" there are of a given file, in most (virtually all?) cases tape-archive instances shouldn't count. So maybe it would be wiser to have separate constructs that are analogous to Store and FileInstance, but that are not the same, so that we don't have to keep on remembering to implement special logic to ignore tape archive things when they're not relevant. It is hard for me to think of use cases where we'll really want to treat regular and tape-archive Stores, or regular and tape-archive Instances, as being on an equal footing.