Current ndbmtd online backup is including too many entries in the
backup log.

Specifically, row changes occurring to fragments managed by LDM
instance 1 are always recorded in the log, even if they are for
non-primary fragments.

This wastes resources at restore time, and can result in other
problems when e.g. staging tables are used for schema transforms
during ndb_restore.

This patch corrects the problem, and adds an MTR testcase which
checks that the backup log contains only the expected entries,
and no duplicates.