CBD-792: Set writes back to 20%

Change-Id: I7b5c32b63ac5ff03d29c3bf6617516bbb76d961d
Reviewed-on: http://review.couchbase.org/24563
Reviewed-by: abhinav dangeti <abhinav.dangeti@gmail.com>
Tested-by: abhinav dangeti <abhinav.dangeti@gmail.com>