MB-18447 - regularly ping DCP stream when no recent activity
MB-18447 - regularly ping DCP stream when no recent activity

The hope here is that the occasional ping (or sending a NOOP) message
when there's been no recent activity will force the TCP machinery to
either keep the TCP connection alive, or to notice that the connection
actually is closed.

Change-Id: If0a20c14d146802e519dac7e7e8eef4d70f6a256
Reviewed-on: http://review.couchbase.org/63190
Reviewed-by: Marty Schoch <ba9951d7d66412e2a57aea395f01176a793cf5eb@gmail.com>
Tested-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

couchbase/go-couchbase