MB-6108 change aggregate method for view ops stats

from sum to max.
Reason: Each view query hits all nodes.
Thus adding view hits from all nodes multiplies
query rate by times number of nodes if sum is
used to aggregate values between nodes.

Change-Id: Ic9bc502a25071afb8a30f1b7e5cd6742f695f5b8
Reviewed-on: http://review.couchbase.org/28590
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>