Add SQLCipher API to SQLCipher backend.

* Add code for use of raw 64 hex-char key in sqlcipher databases.
* Add encrypted db assertion according to sqlcipher doc.
* Add the following PRAGMAS to the API:
    * PRAGMA cipher
    * PRAGMA kdf_iter
    * PRAGMA cipher_page_size
    * PRAGMA rekey
Encode all u1db data before storing.
Add cipher, hmac and kdf info to docstrings.
Add changes file.
Merge remote-tracking branch 'drebs/feature/2277-add-sqlcipher-api-to-sqlcipher-backend-2' into develop
Merge remote-tracking branch 'drebs/feature/2359-avoid-encoding-problems-when-storing-u1db-data' into develop