cuirass: Perform some database "optimization" at startup.

Add a "optimize" step that occurs when starting up the main Curiass
process. Currently this does two things, but could be extended to do more.

The "PRAGMA optimize;" command prompts SQLite to ANALYZE tables where that
might help. The "PRAGMA wal_checkpoint(TRUNCATE);" command has SQLite process
any unprocessed changes from the WAL file, then truncate it to 0 bytes. I've
got no data to suggest this helps with performance, but I'm hoping that going
from a large WAL file to a small one occasionally might be useful.

* src/cuirass/database.scm (db-optimize): New procedure.
* bin/cuirass.in (main): Run it.