Be healthier about retransmissions

[jordansissel]

When a retransmission occurs, especially because of an ack timeout, it would be best not to retransmit the full spool because if another ack timeout occurs, we'll keep repeating that over and over, retransmitting until we OOM Logstash or starve it of connections.

Given lumberjack is loosely based on TCP window sizing/acknowledgements, it bears noticing that tcp slow-start would help solve this. When needing a retransmission, use slow-start (tiny window size that grows with success).