db: Fix 'obsolete' marking

"obsoleted-by" status is the message-id of newer series.

"obsoleted-by-version" status is the version of new series.

Signed-off-by: Fam Zheng <famz@redhat.com>
Add "syncdb" subcommand.

This allows metadata (reviewed/obsoleted/replied) to be fixed without
re-import everything.

Signed-off-by: Fam Zheng <famz@redhat.com>