Rework how we use storage for storing images

This patch overhauls how we use containers/storage to store images,
dropping most of our own metadata in favor of facilities that are
now provided by default by the storage library.  Additionally:
* storageImageDestination now caches blobs in a temporary directory
  until Commit() is called
* storageImageDestination generates a barebones manifest if one isn't
  supplied before Commit() is called
* storageImageDestination uses new APIs in containers/storage to
  look for a local layer with the same contents of a blob, making it
  better at noticing when a PutBlob() isn't necessary
* storageImageDestination sets the creation date for the image if it
  can be determined during Commit()
* storageImageDestination defaults to using the hex part of the digest
  of the image's configuration blob as an image's ID, making it better
  at catching re-pulls of the same image
* storageImageDestination no longer discards names which have been set
  for an image when reusing an ID
* storageImage now counts sizes of uncompressed data when determining
  image size
* storageImage now counts the size of the configuration blob when
  computing an image's size
* storageImage returns an updated image with the manifest listing
  uncompressed layer blobs
* storageImageSource also returns such an updated manifest
* storageImageSource now always returns uncompressed layers

Test changes:
* storage tests now always write an image manifest
* the test for determining an image's size now actually writes the
  configuration blob that it later tries to read

Signed-off-by: Nalin Dahyabhai <[email protected]>