Use SQLite PRAGMA optimize

[twm]

SQLite 3.18.0 (2017-03-28) introduces PRAGMA optimize which should be run before closing each database connection. Once this becomes available in the SQLite Yarrharr uses add support for running this. Ubuntu 16.04 has 3.11.0 2016-02-15, so this will probably become available in Ubuntu 18.04

Consider:

Applications that desire more control can run PRAGMA optimize(0x03) to obtain a list of ANALYZE commands that SQLite thinks are appropriate to run, but without actually running those commands. If the returned set is non-empty, the application can then make a decision about whether or not to run the suggested ANALYZE commands, perhaps after prompting the user for guidance.

ANALYZE is a full-table scan, so it would be best to avoid running it from a Django WSGI thread (that would negatively impact response latency). Instead, use PRAGMA optimize(0x03) to get the operations to run in a background thread—perhaps the poller thread.