Add persistent congestion TODO.

Summary:
There's a lot of discussion about this in the WG. See:

quicwg/base-drafts#3937

quicwg/base-drafts#3939

quicwg/base-drafts#3875

Our approach is very conservative, which may mean we are declaring persistent congestion mor frequently than we need to. This seems to work okay, but put a TODO to remind us to experiment with this in the future.

Reviewed By: yangchi

Differential Revision: D22699664

fbshipit-source-id: c0b73fc48a9f3dd141cada95eeb8493d17322702

quic/state/AckHandlers.cpp

@@ -202,6 +202,11 @@ void processAckFrame(
       (ack.largestAckedPacket.has_value() || lossEvent)) {
     if (lossEvent) {
       CHECK(lossEvent->largestLostSentTime && lossEvent->smallestLostSentTime);
+      // TODO it's not clear that we should be using the smallest and largest
+      // lost times here. It may perhaps be better to only consider the latest
+      // contiguous lost block and determine if that block is larger than the
+      // congestion period. Alternatively we could consider every lost block
+      // and check if any of them constitute persistent congestion.
       lossEvent->persistentCongestion = isPersistentCongestion(
           conn,
           *lossEvent->smallestLostSentTime,