oohttp: net/http fork to ease using utls

[bassosimone]

We should create a net/http fork to ease using utls. The patch to make net/http accept any TLS library is trivial (I already tested this in a private repository) and the flow of changes is small enough that it should not make merging hard.

The gist of the patch is introducing a TLSConn type in http that implements net.Conn and has the same Handshake and ConnectionState methods of the *tls.Conn struct. Once we have that, we need to replace any usage of *tls.Conn inside the net/http fork with TLSConn. With this change in, we can use refraction-networking/utls.

[bassosimone]

I've started publishing the experimental work I've already done at https://github.com/ooni/oohttp.

[bassosimone]

This work has already been done in Sprint 45. As such, I can close this issue now.