延时队列 DelayQueue 实现

[flycash]

新版本：

• 核心是实现了自己定义的 cond，它能够达成 await(time.Duration) 的效果，简化了代码，代价就是会创建额外的 channel

[codecov]

Codecov Report

Merging #115 (cb55587) into dev (27357ee) will decrease coverage by 0.64%.
The diff coverage is 89.24%.

❗ Current head cb55587 differs from pull request most recent head 4e01073. Consider uploading reports for the commit 4e01073 to get more accurate results

@@            Coverage Diff             @@
##              dev     #115      +/-   ##
==========================================
- Coverage   95.42%   94.78%   -0.65%     
==========================================
  Files          28       29       +1     
  Lines        1269     1362      +93     
==========================================
+ Hits         1211     1291      +80     
- Misses         43       54      +11     
- Partials       15       17       +2     

Impacted Files	Coverage Δ
queue/delay_queue.go	89.24% <89.24%> (ø)
pool/task_pool.go	98.11% <0.00%> (-0.95%)	⬇️

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[flycash]

我觉得从可读性上来说，这个代码最直观也最简洁

[flycash]

还是有破绽=。=

[flycash]

思来想去，还是觉得没有办法在同时控制 ctx 超时的时候，保持代码简单。