Remove direct `Instant::now()` calls

[birneee]

Quiche currently uses Instant::now() directly in various places. Moving time management from quiche to the applications would make the quiche state machine more deterministic, which would improve testability. Also, the time granularity could be controlled by the application, potentially reducing the number of syscalls.

For example the signature of send(&mut self, out: &mut [u8]) could be changed to send(&mut self, out: &mut [u8], now: Instant).

Automated tests that require timed events would be simpler. Example:

     #[test]
     /// Tests that old data is retransmitted on PTO.
     fn early_retransmit() {
         let mut buf = [0; 65535];
+        let mut now = EPOCH;
 
-        let mut pipe = testing::Pipe::new().unwrap();
+        let mut pipe = testing::Pipe::new(now).unwrap();
-        assert_eq!(pipe.handshake(), Ok(()));
+        now = pipe.handshake(now, Duration::from_millis(1)).unwrap();
 
         // Client sends stream data.
         assert_eq!(pipe.client.stream_send(0, b"a", false), Ok(1));
-        assert_eq!(pipe.advance(), Ok(()));
+        now = pipe.advance(now, Duration::from_millis(1)).unwrap();
 
         // Client sends more stream data, but packet is lost
         assert_eq!(pipe.client.stream_send(4, b"b", false), Ok(1));
-        assert!(pipe.client.send(&mut buf).is_ok());
+        assert!(pipe.client.send(&mut buf, now).is_ok());
 
         // Wait until PTO expires. Since the RTT is very low, wait a bit more.
-        let timer = pipe.client.timeout().unwrap();
+        let timer = pipe.client.timeout(now).unwrap();
-        std::thread::sleep(timer + time::Duration::from_millis(1));
+        now += timer + Duration::from_millis(1);

-        pipe.client.on_timeout();
+        pipe.client.on_timeout(now);
 
         let epoch = packet::Epoch::Application;
         assert_eq!(
             pipe.client
                 .paths
                 .get_active()
                 .expect("no active")
                 .recovery
                 .loss_probes(epoch),
             1,
         );
 
         // Client retransmits stream data in PTO probe.
-        let (len, _) = pipe.client.send(&mut buf).unwrap();
+        let (len, _) = pipe.client.send(&mut buf, now).unwrap();
         assert_eq!(
             pipe.client
                 .paths
                 .get_active()
                 .expect("no active")
                 .recovery
                 .loss_probes(epoch),
             0,
         );
 
         let frames =
             testing::decode_pkt(&mut pipe.server, &mut buf[..len]).unwrap();
 
         let mut iter = frames.iter();
 
         // Skip ACK frame.
         iter.next();
 
         assert_eq!(
             iter.next(),
             Some(&frame::Frame::Stream {
                 stream_id: 4,
                 data: stream::RangeBuf::from(b"b", 0, false),
             })
         );
         assert_eq!(pipe.client.stats().retrans, 1);
     }