Cleanup and modularize receive path, improve timestamp support [5/x]

Summary:
This diff changes `QuicAsyncUDPSocketWrapper` so that it is an abstraction layer that inherits from `QuicAsyncUDPSocketType`, instead of simply being a container with aliases.
- Key changes in `QuicAsyncUDPSocketWrapper.h`, the rest of the updates switch us from using `QuicAsyncUDPSocketType` to `QuicAsyncUDPSocketWrapper`.
- It's difficult to mock the UDP socket today given that we expose the entire `folly::AsyncUDPSocket` type to the higher layers of the QUIC stack. This complicates testing and emulation because any mock / fake has to implement low level primitives like `recvmmsg`, and because the `folly::AsyncUDPSocket` interface can change over time.
- Pure virtual functions will be defined in `QuicAsyncUDPSocketWrapper` in a follow up diff to start creating an interface between the higher layers of the mvfst QUIC stack and the UDP socket, and this interface will abstract away lower layer details such as `cmsgs` and `io_vec`, and instead focus on populating higher layer structures such as `NetworkData` and `ReceivedPacket` (D48714615). This will make it easier for us to mock or fake the UDP socket.

This diff relies on changes to `folly::MockAsyncUDPSocket` introduced in D48717389.

--

This diff is part of a larger stack focused on the following:

- **Cleaning up client and server UDP packet receive paths while improving testability.** We currently have multiple receive paths for client and server. Capabilities vary significantly and there are few tests. For instance:
  - The server receive path supports socket RX timestamps, abet incorrectly in that it does not store timestamp per packet. In comparison, the client receive path does not currently support socket RX timestamps, although the code in `QuicClientTransport::recvmsg` and `QuicClientTransport::recvmmsg` makes reference to socket RX timestamps, making it confusing to understand the capabilities available when tracing through the code. This complicates the tests in `QuicTypedTransportTests`, as we have to disable test logic that depends on socket RX timestamps for client tests.
  - The client currently has three receive paths, and none of them are well tested.

- **Modularize and abstract components in the receive path.** This will make it easier to mock/fake the UDP socket and network layers.
  - `QuicClientTransport` and `QuicServerTransport` currently contain UDP socket handling logic that operates over lower layer primitives such `cmsg` and `io_vec` (see `QuicClientTransport::recvmmsg` and `...::recvmsg` as examples).
  - Because this UDP socket handling logic is inside of the mvfst transport implementations, it is difficult to test this logic in isolation and mock/fake the underlying socket and network layers. For instance, injecting a user space network emulator that operates at the socket layer would require faking `folly::AsyncUDPSocket`, which is non-trivial given that `AsyncUDPSocket` does not abstract away intricacies arising from the aforementioned lower layer primitives.
  - By shifting this logic into an intermediate layer between the transport and the underlying UDP socket, it will be easier to mock out the UDP socket layer when testing functionality at higher layers, and inject fake components when we want to emulate the network between a mvfst client and server. It will also be easier for us to have unit tests focused on testing interactions between the UDP socket implementation and this intermediate layer.

- **Improving receive path timestamping.** We only record a single timestamp per `NetworkData` at the moment, but (1) it is possible for a `NetworkData` to have multiple packets, each with their own timestamps, and (2) we should be able to record both userspace and socket timestamps.

Reviewed By: jbeshay, hanidamlaj

Differential Revision: D48717388

fbshipit-source-id: 4f34182a69ab1e619e454da19e357a6a2ee2b9ab

quic/api/QuicBatchWriterFactory.cpp

@@ -97,11 +97,11 @@ class ThreadLocalBatchWriterCache : public folly::AsyncTimeout {
       if (evb && evb->getBackingEventBase() && !socket_) {
         auto fd = writer->getAndResetFd();
         if (fd >= 0) {
-          socket_ = std::make_unique<quic::QuicAsyncUDPSocketType>(
+          socket_ = std::make_unique<quic::QuicAsyncUDPSocketWrapper>(
               evb->getBackingEventBase());
           socket_->setFD(
               quic::toNetworkFdType(fd),
-              quic::QuicAsyncUDPSocketType::FDOwnership::OWNS);
+              quic::QuicAsyncUDPSocketWrapper::FDOwnership::OWNS);
         }
         attachTimeoutManager(evb->getBackingEventBase());
       }
@@ -128,7 +128,7 @@ class ThreadLocalBatchWriterCache : public folly::AsyncTimeout {
       quic::QuicBatchingMode::BATCHING_MODE_NONE};
   // this is just an  std::unique_ptr
   std::unique_ptr<quic::BatchWriter> batchWriter_;
-  std::unique_ptr<quic::QuicAsyncUDPSocketType> socket_;
+  std::unique_ptr<quic::QuicAsyncUDPSocketWrapper> socket_;
 };
 #endif
 } // namespace