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!)