Imp: remove client from pool before closing it

[AlexStocks]

What this PR does:

remove client from pool before closing it to ensure other task can not got it.

[codecov-io]

Codecov Report

Merging #190 into develop will increase coverage by 1.01%.
The diff coverage is 60%.

@@             Coverage Diff             @@
##           develop     #190      +/-   ##
===========================================
+ Coverage     66.8%   67.82%   +1.01%     
===========================================
  Files           94       94              
  Lines         5862     5864       +2     
===========================================
+ Hits          3916     3977      +61     
+ Misses        1535     1474      -61     
- Partials       411      413       +2

Impacted Files	Coverage Δ
protocol/dubbo/pool.go	73.6% <60%> (+16.16%)	⬆️
protocol/dubbo/readwriter.go	74.35% <0%> (+2.56%)	⬆️
protocol/dubbo/codec.go	82.35% <0%> (+8.82%)	⬆️
protocol/dubbo/listener.go	64.81% <0%> (+10.64%)	⬆️

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[AlexStocks]

If there is no problems, pls merge it. @hxmhlt @fangyincheng

[hxmhlt]

LGTM

[fangyincheng]

LGTM