Use 'uint32' instead of 'int64' for blob sizes everywhere.

Not just in blob.SizedRef, but in blobserver.Fetch and
blobserver.FetchStreaming, too.
Blobs have a max size of 10-32 MB anyway, and the index.Corpus is now using
uint32 to save memory.

Change-Id: I1172445c2f9463fdaee55bfe0f1218d44be4aa53
appengine: fix build. Use uint32 instead of int64 for blob sizes.

Change-Id: I1952e291b8d86dfaed8310bd5812688849225435