hides overloaded virtual function

On clang7

```
[2020-06-09T02:55:09.278Z] /work/mxnet/3rdparty/ps-lite/include/ps/kv_app.h:417:8: error: 'ps::KVServer::set_request_handle' hides overloaded virtual function [-Werror,-Woverloaded-virtual]
[2020-06-09T02:55:09.278Z] void set_request_handle(const ReqHandle& request_handle) {
[2020-06-09T02:55:09.278Z] ^
[2020-06-09T02:55:09.278Z] /work/mxnet/src/kvstore/./kvstore_dist_server.h:159:22: note: in instantiation of template class 'ps::KVServer' requested here
[2020-06-09T02:55:09.278Z] ps_server_ = new ps::KVServer(0);
[2020-06-09T02:55:09.278Z] ^
[2020-06-09T02:55:09.278Z] /work/mxnet/3rdparty/ps-lite/include/ps/simple_app.h:83:23: note: hidden overloaded virtual function 'ps::SimpleApp::set_request_handle' declared here: type mismatch at 1st parameter ('const ps::SimpleApp::Handle &' (aka 'const function &') vs 'const ps::KVServer::ReqHandle &' (aka 'const function &, ps::KVServer *)> &'))
[2020-06-09T02:55:09.278Z] virtual inline void set_request_handle(const Handle& request_handle) {
[2020-06-09T02:55:09.278Z] ^
[2020-06-09T02:55:09.278Z] In file included from /work/mxnet/src/kvstore/kvstore.cc:25:
[2020-06-09T02:55:09.278Z] In file included from /work/mxnet/include/mxnet/kvstore.h:37:
[2020-06-09T02:55:09.278Z] In file included from /work/mxnet/3rdparty/ps-lite/include/ps/ps.h:13:
[2020-06-09T02:55:09.278Z] /work/mxnet/3rdparty/ps-lite/include/ps/kv_app.h:427:8: error: 'ps::KVServer::Response' hides overloaded virtual function [-Werror,-Woverloaded-virtual]
[2020-06-09T02:55:09.278Z] void Response(const KVMeta& req, const KVPairs& res = KVPairs());
[2020-06-09T02:55:09.278Z] ^
[2020-06-09T02:55:09.278Z] /work/mxnet/3rdparty/ps-lite/include/ps/simple_app.h:69:23: note: hidden overloaded virtual function 'ps::SimpleApp::Response' declared here: type mismatch at 1st parameter ('const ps::SimpleData &' vs 'const ps::KVMeta &')
[2020-06-09T02:55:09.278Z] virtual inline void Response(const SimpleData& recv_req, const std::string& res_body = "");
[2020-06-09T02:55:09.278Z] ^
[2020-06-09T02:55:09.278Z] 2 errors generated.
```

triggered by https://github.com/apache/incubator-mxnet/pull/18513