MB-8662: changed default deletions purge interval to 3

As requested by Anil

Change-Id: I631257f0fb6f866b1c6f05be21bc2ab6fa8be4a9
Reviewed-on: http://review.couchbase.org/28088
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>