fix deadlock #57

[ykalayy]

Fix #57

1. closed state access and updates are wrapped with get/set functions. To eliminate possible deadlocks on pool close

[ykalayy]

@jolestar Could you please review the PR?

[jolestar]

LGTM, thanks

[ykalayy]

@jolestar I forget to stop testTimeoutTicker in UT. and Ofc, It is affected other tests in the suit. I updated.
Added timer stop in UT:

defer testTimeoutTicker.Stop()

Could you review it again, please?

[ykalayy]

Added ticker stop in UT:

defer ticker.Stop()

[codecov]

Codecov Report

Merging #58 (6c285b9) into master (1d53859) will increase coverage by 0.25%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   91.75%   92.01%   +0.25%     
==========================================
  Files           8        8              
  Lines         898      902       +4     
==========================================
+ Hits          824      830       +6     
+ Misses         40       39       -1     
+ Partials       34       33       -1     

Impacted Files	Coverage Δ
pool.go	91.53% <90.90%> (+0.64%)	⬆️

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