Don't cache deserialised incremental stat state in partition object

Before this patch, HdfsPartition would eagerly decode the
TPartitionStats object from the HMS parameter map during
construction. This leads to double the memory cost, as the parameter map
version is always retained in memory as well. This patch changes that
behaviour to only decode when needed, and then only into a temporary
object. This cuts the memory overhead of incremental stats by about
50%. Since the object is only used during incremental stats computation,
trading a (very) little latency for this much memory is well worth it.

Change-Id: I657e11f15ff2e6d0c17f5686e6270c9a0f055a8e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5435
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins