Fix race condition in connection termination

See hyperium/hyper#3652.

What I have found is the final reference to a stream being dropped
after the `maybe_close_connection_if_no_streams` but before the
`inner.poll()` completes can lead to the connection dangling forever
without any forward progress. No streams/references are alive, but the
connection is not complete and never wakes up again. This seems like a
classic TOCTOU race condition.

In this fix, I check again at the end of poll and if this state is
detected, wake up the task again.

Wth the test in hyperium/hyper#3655, on my machine, it fails about 5% of the time:
```
1876 runs so far, 100 failures (94.94% pass rate). 95.197349ms avg, 1.097347435s max, 5.398457ms min
```

With that PR, this test is 100% reliable
```
64010 runs so far, 0 failures (100.00% pass rate). 44.484057ms avg, 121.454709ms max, 1.872657ms min
```

Note: we also have reproduced this using `h2` directly outside of `hyper`, which is what gives me
confidence this issue lies in `h2` and not `hyper`.

src/client.rs

@@ -1428,7 +1428,12 @@ where
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         self.inner.maybe_close_connection_if_no_streams();
-        self.inner.poll(cx).map_err(Into::into)
+        let result = self.inner.poll(cx).map_err(Into::into);
+        if result.is_pending() && !self.inner.has_streams_or_other_references() {
+            tracing::trace!("last stream closed during poll, wake again");
+            cx.waker().wake_by_ref();
+        }
+        result
     }
 }
 

src/proto/connection.rs

@@ -242,6 +242,13 @@ where
         }
     }
 
+    /// Checks if there are any streams or references left
+    pub fn has_streams_or_other_references(&self) -> bool {
+        // If we poll() and realize that there are no streams or references
+        // then we can close the connection by transitioning to GOAWAY
+        self.inner.streams.has_streams_or_other_references()
+    }
+
     pub(crate) fn take_user_pings(&mut self) -> Option<UserPings> {
         self.inner.ping_pong.take_user_pings()
     }