Streaming: Single retransmit timer per connection (ticket #2715)

Only instantiate ResendPacketEvent for fast retransmit
Move packet timeout field from ResendPacketEvent to PacketLocal
Set window size to 1 on timeout
Always adjust window when acked, even if the packet was sent more than once
Reduce INITIAL_WINDOW_SIZE from 6 to 3 to reduce router DH overhead
Set maximum burst retransmissions to 16
Closer compliance to RFCs 5681 and 6298
Prep for additional changes (Westwood)
Other minor cleanups
Original patch from zlatinb