Feature/soledad adbapi3

DO NOT MERGE YET
just for testing / pullreq purposes

Would something like this, mixing both scenarios, work?

1. derive a unique id from username and pass -> used as doc_id
2. derive a second unique id from username and pass, and xor the username with it when creating the recovery db. store the secrets document with this (doc_recovery_user)
3. when retrieving the db, send this second token too.
4. server gets the recovery token, and xors the doc_recovery_user with it.
5. server only sends the recovery document if xor(token, doc_recovery_user) == auth_user