opt: move taskpool at endpoint layer

[watermelo]

What this PR does:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[wongoo]

using go mod tidy to remove unused lib version in go.sum

[AlexStocks]

move out to 3rd import block.

[watermelo]

done

[codecov-io]

Codecov Report

Merging #50 (9ae631e) into master (bf36b5d) will decrease coverage by 0.21%.
The diff coverage is 12.50%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   66.07%   65.85%   -0.22%     
==========================================
  Files           8        8              
  Lines        1350     1356       +6     
==========================================
+ Hits          892      893       +1     
- Misses        365      370       +5     
  Partials       93       93              

Impacted Files	Coverage Δ
client.go	67.41% <0.00%> (-0.31%)	⬇️
options.go	81.25% <0.00%> (-11.61%)	⬇️
session.go	65.15% <0.00%> (ø)
server.go	65.86% <100.00%> (+0.13%)	⬆️

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[watermelo]

using go mod tidy to remove unused lib version in go.sum

done

[AlexStocks]

流程：
1 退出；
2 如果 p.work 没有任务，就会阻塞，执行 t；
3 再次尝试 step2
4 如果设定的task 数目没有超过， 则新启动一个 goroutine 执行 worker；
5 所有 work gr 都在忙，自动启动一个临时的 gr
就是会有 len(p.sem) 个常驻 gr 构成一个 gr pool，gr pool 没有空闲就启动临时 gr。再也没有阻塞的问题了。

而且以往每个session一个gr pool，现在这个 gr pool 变成全局的了。

所以我建议 gr num也就是 sem len，设置长度是 GOMAXPROC * 200。