grab top keys less frequently

I.e. to make cpu overhead even lower. This changes top keys refresh
interval from 5 seconds to 15 seconds.

Change-Id: I744bf98f1bc5d1fe5bb291c8740f07426963f04f
Reviewed-on: http://review.couchbase.org/38892
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>