Directory sessions table is growing without limit

[amrc-benmorrow]

The session and schema_used tables in the Directory are growing without limit. #614 attempted to fix this but caused other issues with the database structure. The question needs a bit more thought.

There is an endpoint to expose the historical record but it has been disabled for a long time, so currently the only reason for keeping the historical data is so we can do change-notify. We can't work out what has changed without a record of the previous state. But the code and database schema here are fragile, and Directory MQTT change-notify has never been used, despite having been part of the F+ service system since its inception.

Options here are:

• Fix Fix unbounded session table growth in directory #614 so it works correctly, keeping the session table but trimming it when we've done notify.
• Keep some historical record but trim the table to the last month or so. Reinstate the history endpoint.
• Remove change-notify. Remove the session table in favour of keeping current state only.
• Move the Directory state into the ConfigDB. Work out some sensible way to use the classification facilities to provide useful change-notify via the ConfigDB channels.