FEAT: [CADT] Only announce a new mirror if .dat files exist for the singleton you are announcing a mirror for

Mirrors currently get announced the second datalayer subscribes to a store ID. This can cause data propagation to slow, since the newly announced mirror may be checked to receive a .dat file it hasn't had time to download itself.

We should only announce a mirror if a certain amount of .dat files are present on the local device for the specified singleton.