Running goroutines with pullLayer are blocked to send error of a
pull operation. If we abort pulling without notify them about
cancelation they will get stucked forever. To avoid this possible
leak cancelCh was introduced. In case of abort we close that channel
to notify other goroutines about cancelation.
A layer can only be pushed/pulled if the layer preceding it by the
length of the push/pull window has been successfully pushed.
An error returned from pushing or pulling any layer will cause the full
operation to be aborted.
These methods rely on an ObjectStore interface, which is meant to
approximate the storage behavior of the docker engine. This is very much
subject to change.
