[ale] Flush disk sink buffer when sync is called.

Change-Id: I90840b4bf597f4e2e55d84ff6fa2f84a374e7076
Reviewed-on: http://review.couchbase.org/39365
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>