Ensure we can probe all available bandwidth in one ProbeBW_UP

Summary:
When deciding whether its time to switch from ProbeBW_UP to down, the cwndBytes_ value is already limited by the inflightHi_.

This improves the logic by waiting for a target inflight that is 1.25x the current measured BDP and making sure we've hit it (i.e. we've been recently cwndLimited).

We will also exit ProbeBW_Up if the loss level crosses the threshold at any point.

Reviewed By: mjoras

Differential Revision: D68656316

fbshipit-source-id: c95c00e8e7b922dd22e07d58f8bda0ed6ee850b3

quic/congestion_control/Bbr2.cpp

@@ -610,7 +610,7 @@ bool Bbr2CongestionController::checkTimeToCruise() {
 
 bool Bbr2CongestionController::checkTimeToGoDown() {
   if (cwndLimitedInRound_ && inflightHi_.has_value() &&
-      cwndBytes_ >= inflightHi_.value()) {
+      getTargetInflightWithGain(1.25) >= inflightHi_.value()) {
     resetFullBw();
     fullBw_ = maxBwFilter_.GetBest();
   } else if (fullBwNow_) {