bug:use mysql 

storage/session.go:8-13
``` go
type Session struct {
	ID      int `gorm:"primary_key"`
	Version int
	Phone   string `gorm:"uniqueIndex"`
	Data    []byte
}
```
to
```go
type Session struct {
	ID      int `gorm:"primary_key"`
	Version int
	Phone   string `gorm:"uniqueIndex;not null;type:varchar(100)"`
	Data    []byte
}
```
