MB-11735: Rename total_free_bytes -> total_free_mapped_bytes; add total_free_unmapped_bytes

Change-Id: I9efa357adbeff05d993c0602ad49877b3c0bce98
Reviewed-on: http://review.couchbase.org/39874
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>