Fix gnet.Conn not compatible with net.Conn on method Close

[0-haha]

net.Conn 的Close() method的function signature是Close() error, 而gnet.Conn的Close() method的function signature是Close(callback AsyncCallback) error
现在把老的函数重命名成CloseWithCallback(callback AsyncCallback)

name: Fix gnet.Conn not compatible with net.Conn on method Close
about: Propose changes to the code
title: ''
labels: ''
assignees: ''

1. Are you opening this pull request for bug-fixes, optimizations or new feature?

Yes

2. Please describe how these code changes achieve your intention.

3. Please link to the relevant issues (if any).

4. Which documentation changes (if any) need to be made/updated because of this PR?

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[codecov]

Codecov Report

Merging #348 (9654480) into dev (86b3499) will decrease coverage by 0.19%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##              dev     #348      +/-   ##
==========================================
- Coverage   72.50%   72.31%   -0.20%     
==========================================
  Files          13       13              
  Lines        1371     1376       +5     
==========================================
+ Hits          994      995       +1     
- Misses        308      313       +5     
+ Partials       69       68       -1     

Flag	Coverage Δ
unittests	72.31% <71.42%> (-0.20%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
client.go	50.33% <0.00%> (ø)
gnet.go	74.56% <ø> (ø)
connection.go	69.73% <83.33%> (-0.84%)	⬇️

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[panjf2000]

Thanks