MB-100: separated "expensive" heartbeat status building...

into dedicated process.

So that anything things like rare filesystem access or stats access
cannot cause missed heartbeat and therefore false-positive
autofailover. Particularly, we know that with large count of buckets
and heavier loaded system, some of that information can take longer to
gather.

There's now separate process that listens to status update requests
from main heartbeat process. And heartbeat process is only willing to
wait for fresh slow status for heartbeat interval. In case slow status
update takes longer than that, heartbeat sends last seen stale slow
status (with indication how stale it is).

There's now new system_stats histogram to track slow status gathering
latencies: status_latency.

Change-Id: I38cccda3e469f831a1dbdedc416c34a4630e7fb1
Reviewed-on: http://review.couchbase.org/30147
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>