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