MB-100: ask all buckets's warmup-ness in parallel

I.e. so that ns_heart doesn't risk missing autofailover deadlines if
there are many buckets.

Change-Id: If008e03742080d0bb95215656b52b572c9884339
Reviewed-on: http://review.couchbase.org/30122
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>