Add a mechanism to check availability of private data

When a peer is bootstrapped from a snapshot, it may have a significant amount of private to reconcile from other peers. In such a situation it may not be clear to admins when is it safe to delete the existing peer if the intention of bootstrapping a peer was to delete the existing peer(s) to save on storage.

It would be good to add an API for giving the summary of missing pvtdata. It would be good to have this API in general as well for a regular peer because unlike public data, any errors in receiving the private data are mostly silent and are just available in the logs.