add replication latency test with high rate of write operations

Change-Id: I390c4818806f9e9046d1cd511c2c5a4fe3f3ad18
Reviewed-on: http://review.couchbase.org/39238
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>