MB-11582: Increase mutation memory threshold to 98% of the bucket mem quota

Backfill memory threshold still at 95% of the bucket memory quota, but
configurable at runtime.

Change-Id: I453b9d0bd7ac122ea21d6882ed88c010997413d3
Reviewed-on: http://review.couchbase.org/40126
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>