improve performance by reduce hiddin class on every request

Before: qps 2569.09, rt 5.46ms
After : qps 3565.10, rt 2.79ms
Merge pull request #32 from koajs/performance

improve performance by reduce hiddin class on every request
Release 3.0.0
set expire in cookie value

ensure every session will expire
add session.maxAge

support different max ages depending on the route.