Issue 49487 - Restore function that incorrectly removed by last patch
Bug Description: Turns out we still need ldbm_back_entry_release() as
it's used in opshared.c, and its not trival to try and
move it into the backend code.
Fix Description: Restore ldbm_back_entry_release() and still set the
function pointer in the pblock. Also remove the unused
chaining release function. Also did code cleanup with
comments in opshared.c
relates: https://pagure.io/389-ds-base/issue/49487
Reviewed by: elkris(Thanks!)