Make sure we are going to delete the same blob that was requested
some time in the past.

Compare the cache headers so we can detect if the blob is still the same
since the key may be reused across blob entries.
Merge remote-tracking branch 'origin/pr/222'

Signed-off-by: James Myers <jmyers@pivotallabs.com>
Refactor BuildpackBitsDelete and Blobstore::Client.delete

* BuildpackBitsDelete should always be using the BuildpackBlobstore therefore
  does not need the blobstore name to be passed in.
* Consolidate the deletion of blobs into the Blobstore client

[#70734028]

Signed-off-by: James Myers <jmyers@pivotallabs.com>
Merge branch 'master' of https://github.com/cloudfoundry/cloud_controller_ng