🌈 sessions were re-written, update to 4.0.0-alpha.2, read HISTORY.md

**Sessions were re-written **

- Developers can use more than one 'session database', at the same time,
to store the sessions
- Easy to develop a custom session database (only two functions are
required (Load & Update)), [learn
more](https://github.com/iris-contrib/sessiondb/blob/master/redis/database.go)
- Session databases are located
[here](https://github.com/iris-contrib/sessiondb), contributions are
welcome
- The only frontend deleted 'thing' is the: **config.Sessions.Provider**
- No need to register a database, the sessions works out-of-the-box
- No frontend/API changes except the
`context.Session().Set/Delete/Clear`, they doesn't return errors
anymore, btw they (errors) were always nil :)
- Examples (master branch) were updated.

```sh
$ go get github.com/iris-contrib/sessiondb/$DATABASE
```

```go
db := $DATABASE.New(configurationHere{})
iris.UseSessionDB(db)
```

> Note: Book is not updated yet, examples are up-to-date as always.

context_test.go

@@ -596,7 +596,7 @@ func TestContextSessions(t *testing.T) {
 
 	// test destory which also clears first
 	d := e.GET("/destroy").Expect().Status(StatusOK)
-	d.JSON().Object().Empty()
+	d.JSON().Null()
 	// 	This removed: d.Cookies().Empty(). Reason:
 	// httpexpect counts the cookies setted or deleted at the response time, but cookie is not removed, to be really removed needs to SetExpire(now-1second) so,
 	// test if the cookies removed on the next request, like the browser's behavior.