SYSDB: Search the timestamp caches in addition to the sysdb cache
    
    When a sysdb entry is searched, the sysdb cache is consulted first
    for users or groups. If an entry is found in the sysdb cache, the
    attributes from the timestamp cache are merged to return the full and
    up-to-date set of attributes.
    
    The merging is done with a single BASE search which is a direct lookup
    into the underlying key-value database, so it should be relatively fast.
    More complex merging is done only for enumeration by filter which is
    currently done only via the IFP back end and should be quite
    infrequent, so I hope we can justify a more complex merging there.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>