MB-11689 Added a stat ep_meta_data_disk

This gives approx disk space used to store item meta data by couchstore.

Change-Id: I7f1642279c9aca84b02c885fd621cfdf50c20e98
Reviewed-on: http://review.couchbase.org/40038
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>