Introduce PMTU as a variable

Summary:
First step towards d6d. Semantically we need to separate the old `udpSendPacketLen` into `peerMaxPacketSize` as well as `currPMTU`. The former is directly tied to the peer's max_packet_size transport parameter whereas the second is controlled by d6d. To get the actual udp mss, call `conn_->getUdpSendPacketLen()`, which will use the minimum of the two if d6d is enabled, otherwise it will fallback to use `peerMaxPacketSize` only.

During processClientInitialParams and processServerInitialParams, we no longer need to check whether `canIgnorePathMTU` is set because that logic is moved to `setUdpSendPacketLen`. If d6d is enabled, we set both `peerMaxPacketSize` and `currPMTU` to `packetSize` because receiving an initial packet of size x indicates both that the peer accepts x-sized packet and that the PMTU is at least x.

Many call sites and tests are changed.

Faebook:
For now, d6d is considered enabled if `canIgnorePathMTU==false` and `turnoffPMTUD==true`. Down the road, from semantic & practical POV at least one of them should be renamed to something like `enableD6D`, since enabling d6d implies turning off PMTUD and that we should not ignore PMTU. We can keep one for the sake of testing.

Reviewed By: mjoras

Differential Revision: D22049806

fbshipit-source-id: 7a9b30b7e2519c132101509be56a9e63b803dc93

quic/api/test/QuicPacketSchedulerTest.cpp

@@ -862,7 +862,7 @@ TEST_F(QuicPacketSchedulerTest, CloningSchedulerWithInplaceBuilderFullPacket) {
 TEST_F(QuicPacketSchedulerTest, CloneLargerThanOriginalPacket) {
   QuicClientConnectionState conn(
       FizzClientQuicHandshakeContext::Builder().build());
-  conn.udpSendPacketLen = 1000;
+  updateUdpSendPacketLen(conn, 1000);
   conn.streamManager->setMaxLocalBidirectionalStreams(10);
   conn.flowControlState.peerAdvertisedMaxOffset = 100000;
   conn.flowControlState.peerAdvertisedInitialMaxStreamOffsetBidiRemote = 100000;