Merge pull request #152 in ZP/z-push from bugfix/ZP-840-ping-loop-if-exporter-state-is-not to develop

* commit '45cd6bec':
  ZP-840 Invalidate the folder stats in case the exporter state can not be found, improved PingableFolders(), refactored folder stat invalidated into a private method.
  ZP-840 On StateNotFoundException of the exporter state generate a fake change so Ping indicates the synchronization of the folder.