[core] Fix the GCS memory usage high issue

It's not because of leak. The root cause is because we allocate more requests when start. This PR fixed it by making the number of call constant.

src/ray/rpc/grpc_server.cc

@@ -148,7 +148,7 @@ void GrpcServer::Run() {
       if (entry->GetMaxActiveRPCs() != -1) {
         buffer_size = entry->GetMaxActiveRPCs();
       }
-      for (int j = 0; j < buffer_size; j++) {
+      for (int j = 0; j < (buffer_size / num_threads_); j++) {
         entry->CreateCall();
       }
     }