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.

# all files in this directory are under GNU General Public License 3.0

# see LICENSE.txt