backupdb.did_create_directory: use REPLACE INTO, not INSERT INTO + ignore error

backupdb.did_create_directory: use REPLACE INTO, not INSERT INTO + ignore error

This handles the case where we upload a new tahoe directory for a
previously-processed local directory, possibly creating a new dircap (if the
metadata had changed). Now we replace the old dirhash->dircap record. The
previous behavior left the old record in place (with the old dircap and
timestamps), so we'd never stop creating new directories and never converge
on a null backup.