Speed up docker/daemon HasBlob() checks
Speed up docker/daemon HasBlob() checks

Change the list of previously-pushed blobs that we keep in a
docker/daemon destination image into a map, to make lookups faster.

Signed-off-by: Nalin Dahyabhai <f562fcb2a0b898bf0be1afc867ea92325d8f825f@redhat.com>

containers/image,masters-of-cats/image,containers/image