add server-side h2c support

[lucifer9]

When v2ray (using h2 transport) and the reverse proxy are on the same machine, maybe there is no need to use TLS between v2ray and the reverse proxy. #1644

[xiaokangwang]

I think this pull request is ready to be merged.

We needs to add this change into the change log and update documents accordingly.

[eycorsican]

看起来代码有点重复了？if/else 里 assign 不同的 server 应该就可以，这是个小的更改，建议 force push 到一个 commit 里。

[xiaokangwang]

我同意 @eycorsican 的观点。

@lucifer9 ， 你是否想按照 @eycorsican 所建议的，将多个commit合并为一个，并通过if/else assign 不同的server来提高代码复用程度呢？ 如果你不愿意的话可以由我来写这部分的代码。

[lucifer9]

@xiaokangwang
no problem.
I can do as @eycorsican said.
I'll close this PR first.