MB-11005 Randomize initial backup interval in stats_archiver.

So that its less likely that all stats_archivers back up at the same
time.

Change-Id: I48866007b0bbd18843ae45c3c1fd2fc29e5455b5
Reviewed-on: http://review.couchbase.org/39000
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>