jemalloc: Report free {un,}mapped size

Implement the two free memory stats - free_mapped_size and
free_unmapped_size for jemalloc.

Change-Id: Id5aa9338b942468cb688fd2a82e1c208b925aeb7
Reviewed-on: http://review.couchbase.org/41486
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>