ListMultipartUploads: support UploadIdMarker

[halkyon]

This is a spin off from #207.

UploadIdMarker is used to filter out all uploads that come before it in ListMultipartUploads responses.

We do not support this parameter when making the request, and ignore it. We do support key marker, however, as the metainfo API uses the upload key as the cursor for project.ListUploads.

From the ListMultipartUploads API docs:

Together with key-marker, specifies the multipart upload after which listing should begin. If key-marker is not specified, the upload-id-marker parameter is ignored. Otherwise, any multipart uploads for a key equal to the key-marker might be included in the list only if they have an upload ID lexicographically greater than the specified upload-id-marker.