Add unit test of atomicity.
Use a write batch for DB upgrades.

 - the file is encrypted with a master key
 - the master key is encrypted with one or several passwords
 - passwords can be added to or removed from the list
 - a hmac of the file is written at the beginning of the file
 - passwords can be checked/updated without having to read/write the whole file
 - the GUI assumes a single password
 - partial writes:
    encrypted blobs are appended at the end of the file
    each blob is prefixed by its length
    can recover from incomplete write: hmac is updated after appending

 - always commit change to disk, unless we are in a write_batch
 - if we are in a write_batch, commit to disk and to memory at the same time
 - remove wallet.save_db, json_db.set_modified

As a result, db_upgrades now require one write batch per upgrade

Also, raise an exception if we try to write when the DB is closed.
Note that the sanity check in test_storage_upgrade was removed,
because it side effects the storage: calling wallet.stop() closes the DB.