enh: taosConn add atomic closed flag

[yinheli]

taosConn add atomic closed flag to prevent panic issues in concurrent scenarios issue: #228

[yinheli]

Hi @huskar-t I wanted to gently follow up on this PR. Would you have some time to review it when you get a chance?

Also, I've been contemplating on the classification of this PR, would it be more fitting to categorize it as an enhancement or a bug fix? I'd appreciate your insights on this.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 25.00000% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 79.15%. Comparing base (cff3f5e) to head (c7c8d2e).

Files	Patch %	Lines
taosWS/connection.go	40.00%	3 Missing and 3 partials ⚠️
taosWS/statement.go	0.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              3.0     #265      +/-   ##
==========================================
- Coverage   79.16%   79.15%   -0.01%     
==========================================
  Files          67       67              
  Lines        7204     7220      +16     
==========================================
+ Hits         5703     5715      +12     
- Misses        965      966       +1     
- Partials      536      539       +3     

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[huskar-t]

LGTM

[huskar-t]

@menshibin please help review