optimize: close the connection when there are no active requests after the connpool has been shutdown.

[doujiang24]

we implemented the OnGoAway method and call it in pool shutdown.

we can reuse it for graceful shutdown client connection in #1922

[codecov]

Codecov Report

Merging #1967 (7df8b4f) into master (8d97681) will increase coverage by 0.04%.
The diff coverage is 30.00%.

@@            Coverage Diff             @@
##           master    #1967      +/-   ##
==========================================
+ Coverage   61.03%   61.07%   +0.04%     
==========================================
  Files         410      410              
  Lines       35574    35576       +2     
==========================================
+ Hits        21711    21728      +17     
+ Misses      11711    11692      -19     
- Partials     2152     2156       +4     

Impacted Files	Coverage Δ
pkg/stream/xprotocol/connpool_multiplex.go	52.70% <30.00%> (-2.02%)	⬇️
...aulttolerance/regulator/invocation_stat_factory.go	85.00% <0.00%> (-15.00%)	⬇️
...ream/faulttolerance/regulator/default_work_pool.go	88.63% <0.00%> (-4.55%)	⬇️
pkg/metrics/shm/zone.go	66.66% <0.00%> (-2.57%)	⬇️
pkg/module/http2/transport.go	78.47% <0.00%> (-0.44%)	⬇️
pkg/module/http2/server.go	81.70% <0.00%> (-0.15%)	⬇️
pkg/stream/xprotocol/conn.go	40.33% <0.00%> (+2.20%)	⬆️
pkg/stream/xprotocol/connpool_pingpong.go	61.90% <0.00%> (+12.16%)	⬆️
pkg/stream/stream.go	46.66% <0.00%> (+13.33%)	⬆️
...ream/faulttolerance/regulator/default_regulator.go	100.00% <0.00%> (+18.18%)	⬆️

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