Make Storage Driver API calls context aware.

- Change driver interface to take a context as its first argument
     - Make newFileReader take a context as its first argument
     - Make newFileWriter take a context as its first argument
     - Make blobstore exists and delete take a context as a first argument
     - Pass the layerreader's context to the storage layer
     - Pass the app's context to purgeuploads
     - Store the app's context into the blobstore (was previously null)
     - Pass the trace'd context to the storage drivers

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

docs/storage/signaturestore.go

@@ -30,7 +30,7 @@ func (s *signatureStore) Get(dgst digest.Digest) ([][]byte, error) {
 	// can be eliminated by implementing listAll on drivers.
 	signaturesPath = path.Join(signaturesPath, "sha256")
 
-	signaturePaths, err := s.driver.List(signaturesPath)
+	signaturePaths, err := s.driver.List(s.repository.ctx, signaturesPath)
 	if err != nil {
 		return nil, err
 	}