KolabMailSynchronizer: simplified single folder handling when updating UID info
KolabMailSynchronizer: simplified single folder handling when updating UID info

* in kolab_mail_synchronizer_uids_update_infodb(), simplified
  the case that a single folder should be updated
* no more folder summary query needed just to check that a folder
  exists (and to do nothing more with the summary than
  throwing it away)

GNOME/evolution-kolab,GNOME/evolution-kolab