ep_perfsuite(DCP): Change to Mersenne Twister RNG
ep_perfsuite(DCP): Change to Mersenne Twister RNG

Change from default_random_engine to mt19937 for workload
generation. This significantly reduces test runtime - for example the
test 'DCP In-memory (JSON-PADDED)' reduces from 9s to 5s overall test
runtime.

Change-Id: I7e0db743437286f03740fea1b59f35cfbe376d4d
Reviewed-on: http://review.couchbase.org/59603
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

owendCB/ep-engine,daverigby/kv_engine,owendCB/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,membase/ep-engine,hisundar/ep-engine,hisundar/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,hisundar/ep-engine,membase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,owendCB/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,daverigby/ep-engine,daverigby/ep-engine,couchbase/ep-engine,membase/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,membase/ep-engine,couchbase/ep-engine