As discussed in
HDFS-5096, we don't need genstamp and block length when processing cache reports. So let's not send them over the wire (it increases the size of cache reports to 3x what it could be).
Also, we should report the caching statistics alongside the normal DN stats in StorageReport. There's no reason for them to be separate. Since the new fields will be optional and default to 0, there will be no extra overhead in the non-caching case.