MB-9477: allow floating point purge interval

Change-Id: I6f3baca9f717ac1a44d5175c628319efd75f100f
Reviewed-on: http://review.couchbase.org/30108
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>