MB-100: don't crash compaction_daemon on unknown config field

Which complicates adding new compaction setting fields

Change-Id: Ie12267eefac91b5dbf82f7ee74317fdbc7e5fe60
Reviewed-on: http://review.couchbase.org/27858
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>