TCP: Rack ends up sending all that is outstanding every timeout.

In doing some testing for a different problem, I have found rack retransmitting
all outstanding data every time a timeout occurs. The outstanding is sent 1ms
apart between each packet, and then the timeout runs off again. This causes
extra retransmissions when we should be waiting for an ack after sending the
very first segment.

Reviewed by: tuexen
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D36494