MB-100: gather /proc/sys/netstat too

I.e. for low level network statistics like tcp retransmits

Change-Id: Icf80fbac633601cd092b7f9e7447b3fa21a63163
Reviewed-on: http://review.couchbase.org/30329
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>