rle benchmark: make the number of iterations of writing configurable
rle benchmark: make the number of iterations of writing configurable

The benchmark with 1M iterations runs really quickly. Making the number
of iterations configurable helps make it easier to detect subtle differences
in performance.

Change-Id: Id199b2f1a98faeb0112558b87f1a484af9d29665
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1242
Tested-by: jenkins
Reviewed-by: Colin McCabe <3142ce8a0bdba4a38073365893e6b74f72d9c512@cloudera.com>

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