MB-12067: Allow runtime adjustment of defrag params

Allow defragmenter_{interval,age_threshold,chunk_duration} to be
adjusted at runtime, to assist in tuning sensible defaults.

Change-Id: Ifc5a1cd845cebb844d13f7f4fc2264630ad7c0fc
Reviewed-on: http://review.couchbase.org/43053
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Dave Rigby <daver@couchbase.com>