MB-11534: response buffer ack with received package size

Instead of using precalculated buffer size, always use the actual
received package byte size for buffer acknowledgement because
server keeps tracking the total bytes that are sent out.

Change-Id: I3e9613290d5c3f32114a60476b3620c12cc99438
Reviewed-on: http://review.couchbase.org/38843
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>