Cleanup and modularize receive path, improve timestamp support [2/x]

Summary:
This diff:
- Changes `NetworkDataSingle` to have `ReceivedPacket` instead of `Buf`, in line with earlier change to `NetworkData` in D48714615

--

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: mjoras

Differential Revision: D48714796

fbshipit-source-id: d96c2abc81e7c27a01bcd0dd552f274a0c1ede26

quic/api/test/QuicTransportBaseTest.cpp

@@ -265,10 +265,10 @@ class TestQuicTransport
 
   void onReadData(const folly::SocketAddress&, NetworkDataSingle&& data)
       override {
-    if (!data.data) {
+    if (!data.packet.buf) {
       return;
     }
-    folly::io::Cursor cursor(data.data.get());
+    folly::io::Cursor cursor(data.packet.buf.get());
     while (!cursor.isAtEnd()) {
       // create server chosen connId with processId = 0 and workerId = 0
       ServerConnectionIdParams params(0, 0, 0);