[SIP] Shadowsocks v2

This issue is to discuss the changes we want in the next major revision of Shadowsocks protocol. Right now I've done some preliminary research based on the SOCKS6 RFC draft and I have a prototype security layer that provides forward secrecy (except for early data) and 1-RTT latency (or 0-RTT if used with TCP Fast Open).

So here're the things I have in mind (no particular order of importance, and most are optional):

1. v2 protocol roughly based on SOCKS6 (which is still a moving target).
2. New security layer with PFS and 0/1-RTT (w/o TFO). (related issue https://github.com/shadowsocks/shadowsocks-org/issues/54)
3. Basic auth so we can officially support single-port multi-users without hacks.
4. Native solution for DNS. (related issue https://github.com/shadowsocks/shadowsocks-org/issues/156)
5. Better-defined semantics of proxy and VPN regarding errors and ICMP packets (related issue https://github.com/shadowsocks/shadowsocks-org/issues/144)
6. Multiplexing over single TCP connection (similar to HTTP/2) to reduce latency when TFO is not possible.

Please feel free to discuss the changes.

No implementation files or tests are named. Start by reading the linked SOCKS6 RFC draft and the related issues #54, #156, and #144, then review the proposed protocol, security, authentication, DNS, error, and multiplexing changes. Done would require an agreed scope and protocol specification rather than a single implementation edit.