[FLINK-2354] [runtime] Replace old StateHandleProvider by StateStorageHelper in ZooKeeperStateHandleStore

The old StateHandleProvider used in ZooKeeperStateHandleStore had to be replaced because the state backend implementation has changed. Since the new state backend could not be used anymore, a new StateStorageHelper interface has been created. The default implementation FileSystemStateStorageHelper stores the given state onto the specified file system and returns a FileSerializableStateHandle.

Various fixes due to rebasing.