Multiple SST bug fixes and performance improvements.

* Fix connection sequence acks, where some packets would not be acked
  properly and always cause apparent drops and immediate backoffs.
* Also fix handling of first connection packet, which was forced to
  stay in the queued packets so it would be retransmitted if
  necessary, but hadn't been removed upon successful connection. This
  led to it being transmitted again and not acked.
* Properly handle reordering of initial connection sequence
  packets. The previous assumption about proper ordering could cause
  the loss of data as the first data packet would be processed first
  and its data ignored.
* Add slow start support so we can increase the rate much more quickly
  initially. This is a huge performance improvement and gets us to
  pretty sane rates even when there is delay.
* Update SST tests to use larger medium sized payloads and make the
  lossy channel due to congestion since consistent probabilistic
  dropping is unrealistica and causes constant back off.
Fix missing return.