Fix SQLite concurrency issues

A quick test with ApacheBench revealed that SQLite really can't handle
multiple concurrent requests with the default settings, due to a locked
database. This fixes that by following the suggestions here:
https://github.com/mattn/go-sqlite3#faq

Testing with ab -n 100 -c 5 http://localhost:8080/blog/post shows that
this fixes the issue. But we could improve performance by reducing
writes, like what's mentioned in T545.

Part of T529