50215 Logstash No Partial Ack.Fix

Fixed the logstash source to ACK only completed writer windows rather than sometimes emitting a partial ACK before the window is complete. While partial ACKs are permitted by the official protocol spec and cause no problems for the reference go-lumber client in Beats, they appears to confuse proxies that assume there will only be one ACK per window, causing errors on subsequent batches.

authors: bruceg