use Server mode for GC in C# QpsWorker

src/csharp/Grpc.IntegrationTesting.QpsWorker/project.json

@@ -67,5 +67,10 @@
         }
       }
     }
+  },
+  "runtimeOptions": {
+    "configProperties": {
+      "System.GC.Server": true
+    }
   }
 }

templates/src/csharp/Grpc.IntegrationTesting.QpsWorker/project.json.template

@@ -20,5 +20,10 @@
           }
         }
       }
+    },
+    "runtimeOptions": {
+      "configProperties": {
+        "System.GC.Server": true
+      }
     }
   }