Ticket 49097 - fix the pblock to be a hierachial structure
    
    Bug Description:  The pblock was a giant kitchen sink: 712 bytes,
    and analysis showed that normally 80% or more was NULL. We practically
    take RAM and set fire to it wastefully.
    
    Fix Description:  because the pblock is accessed via get/set, we can
    rearrange the internals to a hierachy (tree) of sub-structures that
    are dynamically allocated as required. Each sub structure is a
    grouping of commonly used fields, which aids cache coherrency,
    reduces our memory allocation footprint, and also improves
    our performance as we are allocating and freeing less.
    
    https://pagure.io/389-ds-base/issue/49097
    
    Author: wibrown
    
    Review by: mreynolds (Thanks!)