Imp: split protocol/dubbo/pool.go gettyRPCClient.close and gettyRPCClientPool.remove

[AlexStocks]

What this PR does:
Imp: split protocol/dubbo/pool.go gettyRPCClient.close and gettyRPCClientPool.remove

[codecov-io]

Codecov Report

Merging #186 into develop will decrease coverage by 1.06%.
The diff coverage is 20.68%.

@@            Coverage Diff             @@
##           develop    #186      +/-   ##
==========================================
- Coverage    67.86%   66.8%   -1.07%     
==========================================
  Files           94      94              
  Lines         5850    5862      +12     
==========================================
- Hits          3970    3916      -54     
- Misses        1468    1535      +67     
+ Partials       412     411       -1

Impacted Files	Coverage Δ
protocol/dubbo/pool.go	57.43% <20.68%> (-17.43%)	⬇️
protocol/dubbo/listener.go	54.16% <0%> (-10.65%)	⬇️
protocol/dubbo/codec.go	73.52% <0%> (-8.83%)	⬇️
protocol/dubbo/readwriter.go	71.79% <0%> (-2.57%)	⬇️
protocol/jsonrpc/server.go	51.9% <0%> (-1.15%)	⬇️
registry/etcdv3/registry.go	56.89% <0%> (+1.14%)	⬆️

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[fangyincheng]

LGTM

[hxmhlt]

LGTM