iterator: improve performance of the merging iterator
iterator: improve performance of the merging iterator

Avoid multiplicative indexing into the various buffers --
instead, just bump a pointer as the buffer is marched through.
This yielded a ~15% improvement.

Without patch:

 Performance counter stats for './merge_iterator-test --gtest_break_on_failure --num_iters=10 --num_rows=10000000':

      18047.178653 task-clock                #    0.998 CPUs utilized
             1,605 context-switches          #    0.000 M/sec
                 7 CPU-migrations            #    0.000 M/sec
            69,287 page-faults               #    0.004 M/sec
    47,918,686,591 cycles                    #    2.655 GHz
    16,440,749,059 stalled-cycles-frontend   #   34.31% frontend cycles idle
     7,728,412,058 stalled-cycles-backend    #   16.13% backend  cycles idle
    78,626,533,084 instructions              #    1.64  insns per cycle
                                             #    0.21  stalled cycles per insn
    12,149,600,176 branches                  #  673.213 M/sec
       391,250,844 branch-misses             #    3.22% of all branches

      18.076375328 seconds time elapsed

With patch:

 Performance counter stats for './merge_iterator-test --gtest_break_on_failure --num_iters=10 --num_rows=10000000':

      15501.352946 task-clock                #    0.998 CPUs utilized
             1,390 context-switches          #    0.000 M/sec
                 6 CPU-migrations            #    0.000 M/sec
            69,282 page-faults               #    0.004 M/sec
    41,454,097,335 cycles                    #    2.674 GHz
    11,143,724,802 stalled-cycles-frontend   #   26.88% frontend cycles idle
     7,609,721,897 stalled-cycles-backend    #   18.36% backend  cycles idle
    63,182,416,255 instructions              #    1.52  insns per cycle
                                             #    0.18  stalled cycles per insn
    12,813,365,341 branches                  #  826.597 M/sec
       451,920,797 branch-misses             #    3.53% of all branches

      15.524925375 seconds time elapsed

helifu/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu