check the connection's status when borrow and return

[Nicole00]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

fix #401

Description:

How do you solve it?

related pr #400, thanks @vchangpengfei for proposing the issue and pr.

Special notes for your reviewer, ex. impact of this fix, design document, etc:

[codecov-commenter]

Codecov Report

Base: 63.46% // Head: 63.77% // Increases project coverage by +0.31% 🎉

Coverage data is based on head (b504b45) compared to base (e25c56d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #503      +/-   ##
============================================
+ Coverage     63.46%   63.77%   +0.31%     
- Complexity      787      792       +5     
============================================
  Files            71       71              
  Lines          3758     3760       +2     
  Branches        537      537              
============================================
+ Hits           2385     2398      +13     
+ Misses         1004      994      -10     
+ Partials        369      368       -1     

Impacted Files	Coverage Δ
...com/vesoft/nebula/client/graph/net/NebulaPool.java	87.27% <100.00%> (+0.23%)	⬆️
...vesoft/nebula/client/graph/net/ConnObjectPool.java	56.09% <0.00%> (+2.43%)	⬆️
...java/com/vesoft/nebula/client/meta/MetaClient.java	57.69% <0.00%> (+4.80%)	⬆️

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