Fix: Consumer panic when restart provider

[zouyx]

What this PR does:

• The reason as below :
  When delete provider node will close getty_client and set getty_pool to nil.
  

  dubbo-go/remoting/exchange_client.go

  Line 182 in 1268a96

  client.client.Close()

However ExchangeClient.init is false cause getty_pool will not re-init when re-request.

dubbo-go/remoting/exchange_client.go

Line 61 in 1268a96

init bool

Then getty_pool is nil when re-request, detail as below

Solution：

• Change a flag for re-init getty_pool

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #803 into 1.5 will decrease coverage by 4.16%.
The diff coverage is 52.18%.

@@            Coverage Diff             @@
##              1.5     #803      +/-   ##
==========================================
- Coverage   63.72%   59.56%   -4.17%     
==========================================
  Files         237      258      +21     
  Lines       12295    12585     +290     
==========================================
- Hits         7835     7496     -339     
- Misses       3706     4142     +436     
- Partials      754      947     +193     

Impacted Files	Coverage Δ
cluster/loadbalance/consistent_hash.go	87.75% <ø> (-2.57%)	⬇️
cluster/router/condition/listenable_router.go	54.71% <ø> (-0.13%)	⬇️
cluster/router/tag/file.go	76.19% <ø> (-0.74%)	⬇️
common/extension/health_checker.go	50.00% <ø> (-16.67%)	⬇️
common/extension/metadata_service.go	0.00% <0.00%> (ø)
common/logger/logger.go	91.37% <0.00%> (-3.86%)	⬇️
common/proxy/proxy_factory/default.go	13.33% <0.00%> (-7.31%)	⬇️
common/rpc_service.go	87.94% <ø> (+1.35%)	⬆️
common/yaml/yaml.go	90.90% <0.00%> (-9.10%)	⬇️
config/provider_config.go	50.00% <0.00%> (-8.07%)	⬇️
... and 346 more

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[xibolun]

测试成功