SSU: Redesign of the congestion control (tickets #2412, #2649, #2654, #2713),

modelled on TCP Reno (RFCs 5681 and 6298)
- Use a single timer per connection
- Resend up to half the un-acked messages per timer event instead of a single message
- Only send either old or new messages, do not mix
- Cache/avoid several timer calls
- Instead of 3 return values, allocating bandwidth is now a boolean function
- Avoid one of the iterations over all un-acked messages every packet pusher loop
- Remove 100 ms failsafe
- Fix OMF debug log NPE
With the same cpu usage the bandwidth is much higher
Significant speed improvement for lossy connections (e.g. wifi)
Patch by zlatinb