revAB: Increase workers and iterations

Increase worker threads to 16; and iterations to 6. Assuming the
target cluster has sufficient resource (which ares should have) this
shouldn't take any longer wallclock, but should increase the amount of
'churn' of data and hence expose more fragmentation (hopefully!)

Change-Id: I755e47cd36757820d71aca9fb05e8a1fbf59caed
Reviewed-on: http://review.couchbase.org/40119
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>