backport ITS#9264 to

Hello all,

The fix for the Issue 9264 (Add lock to slapo-unique to delay new ops until
current op is complete) <https://bugs.openldap.org/show_bug.cgi?id=9264>
prevents an inconsistent or unexpected behavior of slapd in the case of
multiple concurrent modifiers, thus I would suggest to backport the fix to
the OPENLDAP_REL_ENG_2_4.

The conditions that trigger this is not that rare and we have encountered
it multiple times, mostly but not only during batch operations

Best Regards,
Nikos