CDK-451: View isReady / signalReady

This PR builds on https://github.com/kite-sdk/kite/pull/98 and https://github.com/kite-sdk/kite/pull/99 adding support for isReady / signalReady on Views.
If this is the only place where ReadySignalable is checked, then it is only checked if the Dataset is Mergeable because this OutputCommitter is only used for Mergeable datasets. I think we need to add this to the commitJob method of the NullOutputCommitter, too. It doesn't matter for the current set of implementations, but it might for new ones, like the remote dataset service.