SyncPTR: Read PTR record values from RBTDB instead of LDAP.
    
    Reading values directly from LDAP was racy because the PTR record value can be
    modified between time-of-check and time-of-use.
    
    This change is not fixing the race completely, SyncPTR needs to be split
    to separate event which will go throught the same event queue as other
    updates to the reverse zone.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/155