Don't respond to a buffer ack unless there is an error

Change-Id: I751d387c3ece85574610dc0b8f4542506b509f89
Reviewed-on: http://review.couchbase.org/38993
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Merge remote-tracking branch 'membase/3.0'

* membase/3.0:
  Don't respond to a buffer ack unless there is an error

Change-Id: I7c07edb70e6fadef6144a283c4a2bc7669f1c335