Optimize timeout statusCode

[CSWYF3634076]

What this PR does:
timeout test bug：https://dubbogoproxy.yuque.com/dubbogoproxy/vld1hq/rfr2mf#gIX74
optimize timeout statusCode
Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:
dubbo 超时返回内容，加入TODO
Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #521 (151f52a) into develop (84affca) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop     #521      +/-   ##
===========================================
+ Coverage    54.68%   54.70%   +0.01%     
===========================================
  Files          666      666              
  Lines        77865    77868       +3     
===========================================
+ Hits         42577    42594      +17     
+ Misses       31653    31640      -13     
+ Partials      3635     3634       -1     

Impacted Files	Coverage Δ
pixiu/pkg/client/dubbo/dubbo.go	21.60% <0.00%> (ø)
pixiu/pkg/client/http/http.go	40.69% <0.00%> (-1.48%)	⬇️
pkg/envoy/admin.go	50.00% <0.00%> (-9.68%)	⬇️
pilot/pkg/config/memory/monitor.go	71.79% <0.00%> (-7.70%)	⬇️
pilot/pkg/model/addressmap.go	63.23% <0.00%> (-4.42%)	⬇️
pkg/envoy/instance.go	76.27% <0.00%> (-1.70%)	⬇️
pkg/istio-agent/xds_proxy.go	60.52% <0.00%> (-1.52%)	⬇️
pilot/pkg/xds/discovery.go	84.93% <0.00%> (-0.97%)	⬇️
...g/serviceregistry/kube/controller/endpointslice.go	73.44% <0.00%> (-0.69%)	⬇️
pilot/pkg/bootstrap/server.go	53.11% <0.00%> (-0.61%)	⬇️
... and 524 more

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

No Coverage information
0.0% Duplication

[mark4z]

LGTM