KolabMailAccess: skip repeated down-sync of recently-synced folders
KolabMailAccess: skip repeated down-sync of recently-synced folders

* when multiple EClients access the same Kolab folder
  (via an ESource and opening of a new view), we sync
  only once and skip syncing for subsequent accesses
  within a short timeframe
* this speeds up initial startup and switching views,
  especially in the light of multiple active EClients,
  which we already have when just starting Evolution
  (at least, the alarm notifier service and Evo itself
  are concurrently accessing the same folders)

GNOME/evolution-kolab,GNOME/evolution-kolab