This commit allows to complete issues 504, 505 and 507 by using the new
import/export/backup/restore interface to get notifications of import
and restore and then resynchronize the server approprietely.

It was necessary to move the call to the import and restore listeners in the
import and restore task implementations because these calls were made too
late and the backend that were given in parameter were already invalid when
the listeners are called.

This code is already covered by the ReSyncTest.java