make onCongestion cca interface

solanaceae/ngc_ft1/cca.hpp

@@ -70,5 +70,8 @@ struct CCAI {
 
 		// if discard, not resent, not inflight
 		virtual void onLoss(SeqIDType seq, bool discard) = 0;
+
+		// signal congestion externally (eg. send queue is full)
+		virtual void onCongestion(void) {};
 };
 

solanaceae/ngc_ft1/flow_only.hpp

@@ -58,8 +58,6 @@ struct FlowOnly : public CCAI {
 
 		void updateWindow(void);
 
-		virtual void onCongestion(void) {};
-
 		// internal logic, calls the onCongestion() event
 		void updateCongestion(void);