[ale] Publish some stats about disk sink.

In particular:

  - compression time
  - total rotation time
  - write time
  - write size

Change-Id: I1f9adb849fce9544de13c41dae1de55a0cb6959f
Reviewed-on: http://review.couchbase.org/38761
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Collect disk sink stats.

Change-Id: I09722ce9eae727963bd2b02ec474999b6d908130
Reviewed-on: http://review.couchbase.org/38762
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>