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

Since for HTTP/2 EWYK has been forced to run in PEC mode, we need the
onFillable() callback to be non blocking.

jetty-http2/http2-common/src/main/java/org/eclipse/jetty/http2/HTTP2Connection.java

@@ -272,7 +272,7 @@ public class HTTP2Connection extends AbstractConnection
         @Override
         public InvocationType getInvocationType()
         {
-            return InvocationType.EITHER;
+            return InvocationType.NON_BLOCKING;
         }
     }
 }