Count cumulative # of egress packets for a stream

Summary:
There are situations where application needs to know how many packets were transmitted for a stream on certain byte range. This diff provides *some* level of that information, by adding the `cumulativeTxedPackets` field in `StreamLike`, which stores the number of egress packets that contain new STREAM frame(s) for a stream.

~~To prevent double-counting, I added a fast set of stream ids.~~

Application could rely on other callbacks (e.g. ByteEventCallback or DeliveryCallback) to get notified, and use `getStreamTransportInfo` to get `packetsTxed` for a stream.

Reviewed By: bschlinker, mjoras

Differential Revision: D23361789

fbshipit-source-id: 6624ddcbe9cf62c628f936eda2a39d0fc2781636

quic/api/QuicTransportFunctions.cpp

@@ -433,6 +433,10 @@ bool handleStreamWritten(
     PacketNumberSpace packetNumberSpace) {
   // Handle new data first
   if (frameOffset == stream.currentWriteOffset) {
+    // Count packet. It's based on the assumption that schedluing scheme will
+    // only writes one STREAM frame for a stream in a packet. If that doesn't
+    // hold, we need to avoid double-counting.
+    stream.numPacketsTxWithNewData++;
     handleNewStreamDataWritten(
         conn, stream, frameLen, frameFin, packetNum, packetNumberSpace);
     return true;