fix: busy loop on shutdown (#834)

When in a slow shutdown, this was busy looping since no more streams were left. The change here now only makes sure we poll one extra time if the state changed during the poll.

Fixes #831

src/client.rs

@@ -1437,8 +1437,14 @@ where
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         self.inner.maybe_close_connection_if_no_streams();
+        let had_streams_or_refs = self.inner.has_streams_or_other_references();
         let result = self.inner.poll(cx).map_err(Into::into);
-        if result.is_pending() && !self.inner.has_streams_or_other_references() {
+        // if we had streams/refs, and don't anymore, wake up one more time to
+        // ensure proper shutdown
+        if result.is_pending()
+            && had_streams_or_refs
+            && !self.inner.has_streams_or_other_references()
+        {
             tracing::trace!("last stream closed during poll, wake again");
             cx.waker().wake_by_ref();
         }