CBD-771: dump compressed stats files

For smaller idle disk utilization.

Change-Id: Ifd93a4b319bd288f9a30ebfad3ec10eea594324f
Reviewed-on: http://review.couchbase.org/24365
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>