The singleflight function was capturing the context.Context of the first
caller that invoked the `singleflight.Do`. This could cause all
concurrent calls to be cancelled when the first request is cancelled.
singleflight calls were also moved from the ImageService to Daemon, to
avoid having to implement this logic in both graphdriver and containerd
based image services.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
