MB-7498: Change high and low watermarks to 85 and 75 respectively

Change-Id: Ie37b79f979b7b7c713ce5a4d8785b2e94245aa33
Reviewed-on: http://review.couchbase.org/23794
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>