MB-10829: confirm consumed data on boundary case too

Socket loop has the logic relies on consumer to confirm reception of
?BUFFER_SIZE div 3 or greater amount of data. But actual code did only >
(without equals).

Particular thing that I saw was that consumer loop had _exactly_
349525 as ConsumedSoFar and socket loop correctly had 349525 as
SentToConsumer (and having previously done {active, false}). In that
state socket loop stopped reading and consumer loop did not ack its
stuff to socket loop which is necessary for socket loop to resume
reading socket.

Change-Id: Id1fe56da585fc37af8a59205f965f1e1220b8687
Reviewed-on: http://review.couchbase.org/35573
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>