CBD-917: increase write rate in dgm xperf config

Change-Id: I3e9135339b3df69aa6da31d4515e2de4b670dee4
Reviewed-on: http://review.couchbase.org/25729
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>