Force the immediate release of reference counted resources on AppEngine

to avoid executors backed by request-scoped threads from becoming
zombies
See
https://cloud.google.com/appengine/docs/java/javadoc/com/google/appengine/api/ThreadManager.html#currentRequestThreadFactory--

core/src/main/java/io/grpc/internal/SharedResourceHolder.java

@@ -141,6 +141,13 @@ public final class SharedResourceHolder {
     Preconditions.checkState(cached.refcount > 0, "Refcount has already reached zero");
     cached.refcount--;
     if (cached.refcount == 0) {
+      if (GrpcUtil.IS_RESTRICTED_APPENGINE) {
+        // AppEngine must immediately release shared resources, particularly executors
+        // which could retain request-scoped threads which become zombies after the request
+        // completes.
+        resource.close(instance);
+        instances.remove(resource);
+      } else {
         Preconditions.checkState(cached.destroyTask == null, "Destroy task already scheduled");
         // Schedule a delayed task to destroy the resource.
         if (destroyer == null) {
@@ -163,6 +170,7 @@ public final class SharedResourceHolder {
           }
         }), DESTROY_DELAY_SECONDS, TimeUnit.SECONDS);
       }
+    }
     // Always returning null
     return null;
   }