make pool simple

[lostz]

type Pool struct {
Dial func() (io.Closer, error)
....
}

func (self *Pool) Get() (io.Closer, error) {
c, err := self.get()
if err != nil {
return nil, err
}
return &pooledConnection{p: self, c: c}, nil
}
这样改写后可以将 pool 变成一个高效通用的连接池.