[ISSUE #42] 没有可用服务时，客户端线程会hang住

[stan9333]

Please provide issue(s) of this PR: Fixes #42

To help us figure out who should review this PR, please put an X in all the areas that this PR affects.

• [ ] Docs
• [ ] Discovery
• [ ] Route
• [ ] RateLimit
• [ ] CircuitBreaker
• [ ] Test and Release

Please check any characteristics that apply to this pull request.

• [ ] Does not have any user-facing changes. This may include API changes, behavior changes, performance improvements, etc.

[codecov[bot]]

Codecov Report

Merging #43 (ea97821) into main (3f6a646) will increase coverage by 0.00%. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              main     #43   +/-   ##
=======================================
  Coverage     8.28%   8.29%           
  Complexity      31      31           
=======================================
  Files           30      30           
  Lines         1002    1001    -1     
  Branches        73      72    -1     
=======================================
  Hits            83      83           
+ Misses         906     905    -1     
  Partials        13      13           

Impacted Files	Coverage Δ
...ent/polaris/grpc/resolver/PolarisNameResolver.java	16.88% <0.00%> (+0.21%)	:arrow_up:

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