MB-8348:added internal setting for use of fadvise in db compactor

Change-Id: I550dfb0b6dd65471437daf40ff4307051f63d8cd
Reviewed-on: http://review.couchbase.org/26527
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>