MB-11735: mem allocator: Report both mapped and unmapped free bytes

From the tcmalloc documentation [1] we see that tcmalloc free memory
is sub-divided into two statistics:

* tcmalloc.pageheap_free_bytes: Number of bytes in free, mapped pages in page heap.
* tcmalloc.pageheap_unmapped_bytes: Number of bytes in free, unmapped pages in page heap.

However at present we only include `pageheap_free_bytes` in the free
calculation.

Rename "total_free_bytes" to "total_free_mapped_bytes", and
"tcmalloc_unmapped_bytes" to "total_free_unmapped_bytes".

Update the calculations of total_fragmentation_bytes to account for
unmapped_bytes.

Change-Id: I3364b732175da25fa9f45490cc4af81e3812836a
Reviewed-on: http://review.couchbase.org/39416
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>