Implement BlobStore UpdateMetadata operations

Multiple blob stores support operations to update the metadata associated with the blob (in S3, this can be done through the PUT-object copy; Swift is already implemented in jclouds; GCS and Azure also use a PUT method). This is preferable to the current approach one has to take with jclouds: get the blob, update metadata, put the blob. If the blob is large, this operation can be prohibitive.