Drop login expiration check from sessions hook.
Fix reference names of incoming request headers. Must be lowercased.
Rewrite sessions hook.

- Simplify code.
- Allow to use custom session TTL in Redis via `env.session_ttl`.
- Add more convenient may to reset sid: env.session_id = null
Remove "general_login_expire_days" setting declaration.

It is moved to Nodeca Users.