moveChunk should attempt to retry write errors during chunk cleanup

Current Mongos will return complete as soon as a chunkMove hit an error in phase 6. It should should retry based on a config.settings.moveRetries=3.

The default would be 0 to preserve previous behavior however this is very helpful to avoid orphans to begin with. I am aware we have a new function to clean them but you can still have logical DB corruption in the mean time.