Fixes flash messaging/SessionCookie decoding failure when cookies.encrypt = true

Removes cookie encryption and decryption from `SessionCookie`.  Puts the responsibility for encryption and decryption back on the app.

**IMPORTANT**: The `SessionCookie` `secret`, `cipher`, and `cipher_mode` settings no longer exist and will have no effect if passed to the `SessionCookie` settings array.  The global `cookies.*` settings are now used for `SessionCookie`.

Supersedes #606.