[FLINK-19296][http] Do not retry HTTP calls on shutdown

This closes #156.
diff --git a/statefun-flink/statefun-flink-core/src/main/java/org/apache/flink/statefun/flink/core/httpfn/RetryingCallback.java b/statefun-flink/statefun-flink-core/src/main/java/org/apache/flink/statefun/flink/core/httpfn/RetryingCallback.java
index 98184d5..ffb7f7b 100644
--- a/statefun-flink/statefun-flink-core/src/main/java/org/apache/flink/statefun/flink/core/httpfn/RetryingCallback.java
+++ b/statefun-flink/statefun-flink-core/src/main/java/org/apache/flink/statefun/flink/core/httpfn/RetryingCallback.java
@@ -87,6 +87,9 @@
   }
 
   private void onFailureUnsafe(Call call, IOException cause) {
+    if (isShutdown.getAsBoolean()) {
+      throw new IllegalStateException("An exception caught during shutdown.", cause);
+    }
     LOG.warn(
         "Retriable exception caught while trying to deliver a message: " + requestSummary, cause);
     metrics.remoteInvocationFailures();