IMPALA-880: COMPUTE STATS should update partitions in batches

When updating partition metadata as part of COMPUTE STATS we would previously
attempt to update all partitions at once. This could lead to HMS socket timeouts
and also could run into issues if there were > 32K partitions.

In this change we now update the partitions in batches, with a max size of 500
partitions per batch. We also compare whether the row count has changed and only
update partitions that have been modified.

Change-Id: If7bfcc30f86fc2fdd79855b981067ac29a47b5e1
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1913
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins