MP2-422: Introduced an abstract ImporterWorkerDataflowBlockBase class which implements all the functionality common to all DataflowBlocks used by ImporterWorkerNewGen. Made the DirectoryUnfoldBlock and the DirectorySaveBlock derive from this class
MP2-422: Solved a bug in CommonNotificationService introduced by the ImporterWorker rework. CommonNotificationService assumed that every MessageType sent by ImporterWorkerMessaging has a ResourcePath argument, which is no longer the case after the introduction of the ImportProgress message. This caused an exception.
MP2-422: Solved an exception in AsyncManualResetEvent that was thrown when the respective task should be cancelled while it already was in a completed state