FIX: [CADT] Only launch mirror once datalayer file is present locally

[ccornick]

We launch mirrors as soon as we subscribe to a coin. The problem with this is that we create a bunch of mirrors that don't actually have the capability to share datalayer files because they have not yet been downloaded from the originator.

So we have different nodes all trying to reach out to these mirrors, timing out before one eventually finds the originator mirror gets the files.

It would be nice if we did a quick check on local DL files to see if we have at least one file available, and THEN launch the mirror for that store.