passdb: Cache output from pdb_[ug]id_to_sid

passdb: Cache output from pdb_[ug]id_to_sid

A customer complained that after upgrading to Samba 4.0 fileserver
its LDAP server was flooded with uid2sid and gid2sid request for id
0. With 4.0 we do a lot more user-space ACL checking which involves
uid2sid/gid2sid. This caches the corresponding results.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Wed Jan  7 12:00:10 CET 2015 on sn-devel-104