r17928: Implement the basic store for CLDAP sitename

r17928: Implement the basic store for CLDAP sitename
support when looking up DC's. On every CLDAP
call store the returned client sitename (if
present, delete store if not) in gencache with
infinate timeout. On AD DNS DC lookup, try looking
for sitename DC's first, only try generic if
sitename DNS lookup failed.
I still haven't figured out yet how to ensure
we fetch the sitename with a CLDAP query before
doing the generic DC list lookup. This code is
difficult to understand. I'll do some experiments
and backtraces tomorrow to try and work out where
to force a CLDAP site query first.
Jeremy.
(This used to be commit ab3f0c5b1e9c5fd192c5514cbe9451b938f9cd5d)