opt: implement the lock-free queue for dispatching tasks faster

[panjf2000]

---

name: Pull request
about: Propose changes to the code
title: ''
labels: ''
assignees: ''

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

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 #181 (1c74459) into master (028bab2) will decrease coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   84.09%   83.81%   -0.29%     
==========================================
  Files          18       18              
  Lines        1182     1180       -2     
==========================================
- Hits          994      989       -5     
- Misses        144      149       +5     
+ Partials       44       42       -2     

Flag	Coverage Δ
unittests	83.81% <100.00%> (-0.29%)	⬇️

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

Impacted Files	Coverage Δ
eventloop_unix.go	66.44% <ø> (-2.59%)	⬇️
eventloop_windows.go	89.65% <ø> (-0.27%)	⬇️
reactor_bsd.go	100.00% <ø> (ø)
reactor_linux.go	86.36% <ø> (-1.64%)	⬇️
server_unix.go	91.60% <100.00%> (+0.36%)	⬆️
server_windows.go	100.00% <100.00%> (ø)

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