Since the IDL-ized ShardsvrMoveRange command is parsing-compatible with the MoveChunkRequest, replace all usages of the latter in order to remove all the duplicated code.

NB: the `writeConcern` is not a member of the ShardsvrMoveRange command, so it will always need to be passed along with the ShardsvrMoveRange. This is the only relevant difference from the MoveChunkRequest that - instead - is doing some dirty magic to get the write concern when deserializing the secondary throttle member.