Patch attached. Straightforward change. I resisted cleaning up BlockTransferThrottler so it's clear it's just a rename to DataTransferThrottler.
Passes all the unit tests. I'm also currently testing setting dfs.image.transfer.bandwidthPerSec to 1mb/sec with a large image to confirm that the rate is in fact throttled and also to test the corner case where the checkpoint period is shorter than the time it takes to transmit the image.