Add target identification support

[martinheidegger]

If I understand this tool correctly, it will stop uploading as soon as the first peer received the whole, updated, drive. This seems like it is going to be a problem if that peer is a regular viewer peer and not a backup node as that may be sparsely replicating: as such the upload progress may stop before everything is uploaded. However, if we could specify one (or maybe several?) remotePublicKeys that need to connected and be verified as "uploaded" then we could make certain that the right peers received a copy of it.

Is that a correct way of thinking about it? Would you be interested in a PR regarding this?

[RangerMauve]

I think that would be super useful!

In tandem it'd be useful for dat-store to announce it's own remotePublicKey on startup or via the / path on the HTTP server so that it'd be easier to identify.