Ticket 49153 - Remove vacuum lock on transaction cleanup
    
    Bug Description:  Previously we held a vacuum lock to prevent
    conflicts during transaction clean up. This created a bottleneck.
    
    Fix Description:  Remove the vacuum lock in favour of a pacman style
    reference count, where each parent holds a refcount to the child, and
    decrements it on free.
    
    https://pagure.io/389-ds-base/issue/49153
    
    Author: wibrown
    
    Review by: mreynolds (Thanks!)