Create a local cache of public keys

[vitorpamplona]

Once a credential is read, its public key should be cached to read future QRs from the same issuer without internet connectivity.

To clarify:

1. Determine the amount of time cache is available without connecting.
2. Determine how to update the cache. (If keys change or it is not available anymore, does it affect past QRs?)
3. Determine how to handle too many records.

[quasistoic]

From thought I've given this in the past, as a user I would like

1. to be able to see a list of the known issuers on my phone,
2. when their keys were last refreshed, and have options
3. to refresh those keys manually
4. or on a schedule when I am connected to the internet
5. and to delete those keys on demand.

In addition, when I encounter a new issuer which is not in my list of known issuers, I would like:

1. to be notified of the new issuer
2. and to be prompted to trust or distrust that issuer.

I know that's a little broader in scope than the narrower sense of this issue, but I think it helps frame an interaction model that would be useful when solving this issue.