use PkgServerClient to choose the best mirror server as pkg server

[johnnychen94]

closes #120

• require New package: PkgServerClient v0.1.0 JuliaRegistries/General#32878

我将镜像站的切换功能重写到 PkgServerClient 里面了，并且增加了自动切换到最近的镜像站的功能：

julia> versioninfo()
Julia Version 1.6.0
Commit f9720dc2eb (2021-03-24 12:55 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.6.0)
  CPU: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)

julia> using JuliaZH

julia> versioninfo()
Julia Version 1.6.0
Commit f9720dc2eb (2021-03-24 12:55 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.6.0)
  CPU: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)
Environment:
  JULIA_PKG_SERVER = https://opentuna.cn/julia

julia> using JuliaZH: PkgServerClient

# 测试镜像站延迟
julia> PkgServerClient.registry_response_time()
Dict{String, Float64} with 8 entries:
  "BFSU"      => 0.106639
  "OpenTUNA"  => 0.335531
  "SJTUG"     => 0.101141
  "USTC"      => 0.0674178
  "SUSTech"   => 0.0906177
  "TUNA"      => 0.114346
  "JuliaLang" => 0.844634
  "NJU"       => 0.0908128

因为是选择延迟最低的镜像站，所以 JULIA_PKG_SERVER 的具体值是不确定的。之前我们是硬编码到了BFSU.

这个PR之后我们需要更新上游镜像站的帮助文档。

[Gnimuc]

off-topic:

Platform Info:
OS: macOS (x86_64-apple-darwin19.6.0)
CPU: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz

你这 CPU 🤩 🥰 😍