Add max I/O time to Hbase runtime stats

Collect and display the maximum Hbase operator I/O time. In a parallel
plan, this is chosen from the biggest Hbase I/O elapsed time among all
the same Hbase tcb instances at the runtime stats merge stage. For
serial plan the value equals to the Hbase I/O elapsed time.

In addition, the displayed Hbase I/O is changed from byte to MB.

Change-Id: Icb4cbcbee5eeaf7143762444d48288f7bf6f5784
Merge "Add max I/O time to Hbase runtime stats"