Set Directory Transport Destination as Thread-Safe

[t0rr3sp3dr0]

Just like #1481 did for OCI Layout Destination, this patch marks Directory Transport Destination as thread-safe. The PutBlobWithOptions implementation of both transports are almost identical, differing only on the path where the blob will be stored.

- 	blobPath := d.ref.layerPath(blobDigest)
+ 	blobPath, err := d.ref.blobPath(blobDigest, d.sharedBlobDir)
+	if err != nil {
+		return types.BlobInfo{}, err
+	}
+	if err := ensureParentDirectoryExists(blobPath); err != nil {
+		return types.BlobInfo{}, err
+	}

[mtrmac]

LGTM. Thanks!