[3.0]remove default session extend

[dsonet]

DESC

Since I use JWT token middle-ware, doesn't need the builtin session.(Because I can't) BTW, I think session should implement as a middle-ware, split as transporter(cookie, header) and storage(File, redis, cookie-in-memory, jwt-in-memory). Then we can unify the usage of JWT and others.

ENV

Platform:

Node.js Version:v8.2

ThinkJS Version:3.0.beta4

[welefen]

多谢建议，我们讨论下。目前 cache, session 是内置的，不过也只是内置了 extend

[welefen]

cache, session 都已经移除