core: Fix DbUserCacheManager when to refresh users

a. firstName of dbUser should be compared & set with firstName of DirectoryUser

Change-Id: Icec0ffa48671e4fed2b631a60db9dd226ad6d347
Bug-Url: https://bugzilla.redhat.com/1081204
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>