core: support completion phase of records

active directory in particular has global catalog with extended
group information available within forest. in order to leverage that we
enable post fetch record completion to give a chance to complete
information before processing.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1134855
Change-Id: I77db14d62698652ea04ccc46e8723aae6c371d01
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>