fix #655 Repeated subscriptions result in multiple callbacks

[yesAnd92]

bug

初始化vo.SubscribeParam的SubscribeCallback属性时是值传递，导致"订阅”和"取消订阅"两次请求传入的回调函数不同，取消订阅逻辑总无法清除对应的callback从而产生这个bug #655。

fix

将SubscribeCallback定义为指针类型的函数，保证"取消订阅"请求时两次回调函数符合相等性的判断。

result

[binbin0325]

use error， you can define the SubscribeParam variable and pass it to Subscribe,Unsubscribe

[binbin0325]

It's not the same problem #655 is trying to solve

[yesAnd92]

Add duplicate check during subscription

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4eb0559) 29.71% compared to head (6fde0a8) 29.93%.
Report is 1 commits behind head on master.

❗ Current head 6fde0a8 differs from pull request most recent head 5cd83e1. Consider uploading reports for the commit 5cd83e1 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #674      +/-   ##
==========================================
+ Coverage   29.71%   29.93%   +0.21%     
==========================================
  Files          40       40              
  Lines        3093     3097       +4     
==========================================
+ Hits          919      927       +8     
+ Misses       2104     2102       -2     
+ Partials       70       68       -2     

Files	Coverage Δ
...s/naming_client/naming_cache/subscribe_callback.go	68.42% <60.00%> (-2.17%)	⬇️

... and 2 files with indirect coverage changes

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[yesAnd92]

@binbin0325 这个pr是否符合要求？pending好久了

[binbin0325]

@binbin0325 这个pr是否符合要求？pending好久了

符合要求，本周再重点看下