fix to release the invalid session for SessionPool

[Nicole00]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

#498

Description:

How do you solve it?

try the session's signOut process, make sure the session release not throw exception but just print the warn log.

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

[codecov-commenter]

Codecov Report

Base: 63.37% // Head: 63.46% // Increases project coverage by +0.09% 🎉

Coverage data is based on head (e6af8a5) compared to base (70d260b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #499      +/-   ##
============================================
+ Coverage     63.37%   63.46%   +0.09%     
  Complexity      787      787              
============================================
  Files            71       71              
  Lines          3754     3758       +4     
  Branches        537      537              
============================================
+ Hits           2379     2385       +6     
+ Misses         1006     1002       -4     
- Partials        369      371       +2     

Impacted Files	Coverage Δ
.../vesoft/nebula/client/graph/SessionPoolConfig.java	49.09% <ø> (ø)
.../com/vesoft/nebula/client/graph/NebulaSession.java	75.00% <100.00%> (+5.00%)	⬆️
...vesoft/nebula/client/graph/net/SyncConnection.java	54.09% <0.00%> (ø)
...va/com/vesoft/nebula/client/graph/SessionPool.java	57.62% <0.00%> (+1.12%)	⬆️

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