*index: increase default --commit-interval to 15s

*index: increase default --commit-interval to 15s

The DBD::SQLite(3pm) sqlite_busy_timeout is 30s and we don't
currently have a way to override this.  Thus 15s should be
adequate assuming we can keep SQLite commit times <10s.
Currently, the worst case commit times can exceed even 30s for
Xapian, but that doesn't affect over.sqlite3 which commits
fairly quickly.