Feature/soledad adbapi3

DO NOT MERGE YET
just for testing / pullreq purposes

I have some thoughts about the shared database I would like to share with you.

The original purpose of the shared database was to allow storage of user recovery documents in server (opt-in/out not yet implemented) in a way resistant to offline attacks: if an attacker has access to the server database, it won't know which recovery document belongs to which user. On the other hand, this currently has the terrible disadvantage that any user can download any recovery document, provided it knows the correct `doc_id`. Supposedly, knowing the `doc_id` is hard because it is a hash of the user's uid and password.

The way I see it, the solution for recovery documents either needs authentication against username to allow for recovery doc download/upload, thus allowing relating users to recovery documents on offline attacks, or else does not need authentication and falls into the case above.

Do you see any better approach for this?