DocSearch: When instanciated, just reload the previous Whoosh index. Update will be done later

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Re-do indexation:
- With an independent thread/worker, examine which documents must be updated
- With a normal thread/worker, update the index

Signed-off-by: Jerome Flesch <jflesch@gmail.com>