pingpong client中的上下文新建，避免和downstream的context混淆

[fibbery]

Issues associated with this PR

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UT result

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[codecov]

Codecov Report

Base: 60.32% // Head: 60.38% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (8e50ffb) compared to base (ea94051).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 8e50ffb differs from pull request most recent head e8b5968. Consider uploading reports for the commit e8b5968 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2232      +/-   ##
==========================================
+ Coverage   60.32%   60.38%   +0.05%     
==========================================
  Files         422      422              
  Lines       37050    37051       +1     
==========================================
+ Hits        22351    22372      +21     
+ Misses      12477    12454      -23     
- Partials     2222     2225       +3     

Impacted Files	Coverage Δ
pkg/stream/http/stream.go	35.23% <100.00%> (+0.13%)	⬆️
pkg/stream/xprotocol/connpool_pingpong.go	62.10% <100.00%> (+12.10%)	⬆️
pkg/upstream/healthcheck/healthchecker.go	88.65% <100.00%> (ø)
...ream/faulttolerance/regulator/default_regulator.go	81.81% <0.00%> (-18.19%)	⬇️
pkg/stream/stream.go	46.66% <0.00%> (-13.34%)	⬇️
pkg/upstream/servicediscovery/dubbod/bootstrap.go	64.58% <0.00%> (-4.17%)	⬇️
pkg/server/handler.go	40.50% <0.00%> (+0.17%)	⬆️
pkg/module/http2/transport.go	78.91% <0.00%> (+0.28%)	⬆️
pkg/module/http2/frame.go	84.47% <0.00%> (+0.30%)	⬆️

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