Replace use of Sequoia's backend with a custom key store.

- Sequoia's key store doesn't meet pep's needs (in particular, the
ability to search on a key's user id) and trying to shoehorn pep's
needs onto Sequoia's key store abstractions is just introducing
overhead with no appreciable gain in functionality.

- This patch changes the Sequoia backend to use a local sqlite
database to store the public keys.

 // This file is under GNU General Public License 3.0

 // see LICENSE.txt

 #pragma once

 #include "transport.h"

 PEP_STATUS email_sendto(PEP_SESSION session, const message *msg);

 PEP_STATUS email_readnext(PEP_SESSION session, message **msg, PEP_transport_t **via);