Implement congestion control from prop324
This implements the Vegas algorithm for congestion control. It moves the current fixed window SENDME code into the "FixedWindow" congestion control algorithm. And finally, uses congestion control in the circuit reactor.
This MR does NOT implement the following which will be done in future MRs:
- NTorv3 extension for CC negotiation
- Flow Control (XON/XOFF)
Ticket #534
Edited by David Goulet