Lua session library for OpenResty / ngx_lua

Features:
 - Sessions are immutable (each save generates a new session), and lockless.
 - Session data is AES-256-GCM encrypted with a key derived using HKDF-SHA256.
 - Session has a fixed size header that is protected with HMAC-SHA256 MAC with a key derived using HKDF-SHA256.
 - Session data can be stored in a stateless cookie or in various backend storages.
 - A single session cookie can maintain multiple sessions across different audiences.