Store sessions in DB

This module changes where Odoo keeps the information about who is logged in. Normally each Odoo server writes session files to its own disk; with this module, sessions are saved in a database table instead.

• It only activates when the environment variable SESSION_DB_URI is set to a database connection address. If that variable is missing, Odoo keeps using its normal file-based sessions.
• When active, it creates a table called http_sessions automatically, holding the session identifier, a last-write date and the session content.
• Logging in, updating a session and logging out write to or delete from that table.
• Old sessions are cleaned up: a routine removes entries older than the standard session lifetime, and helpers can check which session identifiers are missing and delete sessions by identifier prefix.
• It retries a database operation a few times if the connection drops, and uses a lock to keep concurrent access safe.

There are no menus, screens, buttons or reports: it is purely a background configuration change. It does not add any business models and does not touch sales, accounting or inventory data. The visible effect for users is that logins keep working when traffic is spread over several Odoo servers, and administrators no longer need a shared filesystem for session files.