CBSE-889: gather Linux CPU throttling stats

There's some evidence that enabled "EIST" (CPU stepping) feature of
Intel might make a difference between "erlang times out for no
apparent reason" and normal operation. We have been pointed at:
https://bugzilla.kernel.org/show_bug.cgi?id=36182#c27 for more
information. And the ticket mentions this stats, which we now gather
as potential clue about another possible cause of misbehavior.

Change-Id: I59f0f8e1adf7e7d138f9313f328b5945121d103f
Reviewed-on: http://review.couchbase.org/31342
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>