[KuduScanBatch::const_iterator] a minor clean-up

More 'standardized' signature for the prefix increment operator.
Added postfix increment operator.  Changed the behavior of the
equality/non-equality operators: along with current row index,
also check whether iterators are built upon the same batch.
Added units tests to cover the modified code.

Change-Id: I3eeba46e5bc45c9dd7b96abad60173381d6c3e39
Reviewed-on: http://gerrit.cloudera.org:8080/3834
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <todd@apache.org>
2 files changed