Fixes #1719 - Improve handling of HTTP/2 queued requests.

Clearing _waitingForContent in recycle().

jetty-server/src/main/java/org/eclipse/jetty/server/HttpInput.java

@@ -91,6 +91,7 @@ public class HttpInput extends ServletInputStream implements Runnable
             _contentConsumed = 0;
             _firstByteTimeStamp = -1;
             _blockUntil = 0;
+            _waitingForContent = false;
         }
     }