optimize: ignore flushing error when connection is closed or reset

[welkeyever]

What type of PR is this?

optimize

Check the PR title.

• [x] This PR title match the format: <type>(optional scope): <description>
• [x] The description of this PR title is user-oriented and clear enough for others to understand.

(Optional) Translate the PR title into Chinese.

忽略客户端主动断连的写错误

(Optional) More detail description for this PR(en: English/zh: Chinese).

en: change logs level from error to debug if those are the connection error caused by client zh(optional): 将client导致的连接错误日志级别从error调整为debug，进一步减少对用户的打扰信息

Which issue(s) this PR fixes:

None

[codecov[bot]]

Codecov Report

Base: 64.46% // Head: 64.45% // Decreases project coverage by -0.01% :warning:

Coverage data is based on head (6db40b4) compared to base (b4ba993). Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #295      +/-   ##
===========================================
- Coverage    64.46%   64.45%   -0.02%     
===========================================
  Files           84       84              
  Lines         8288     8290       +2     
===========================================
  Hits          5343     5343              
- Misses        2583     2584       +1     
- Partials       362      363       +1     

Impacted Files	Coverage Δ
pkg/network/netpoll/connection.go	89.18% <50.00%> (-5.10%)	:arrow_down:

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