truncate stats every time we start archiver

I.e. so that frequent restarts do not lead to excessive memory and
disk consumption of larger zoom levels. It is believed that CBSE-1089
was caused by it.

Change-Id: I3d74af65cfd28067954bd4264fab0322f67f29a6
Reviewed-on: http://review.couchbase.org/35918
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>