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

#ifdef __cplusplus

extern "C" {

#endif

#ifdef WIN32

#include "platform_windows.h"

#else

#include "platform_unix.h"

#endif

#ifdef __cplusplus

}

#endif