🌈 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.

sessions/provider.go

@@ -1,122 +0,0 @@
-package sessions
-
-import (
-	"container/list"
-	"sync"
-	"time"
-
-	"github.com/kataras/iris/sessions/store"
-)
-
-// IProvider the type which Provider must implement
-type IProvider interface {
-	Name() string
-	Init(string) (store.IStore, error)
-	Read(string) (store.IStore, error)
-	Destroy(string) error
-	Update(string) error
-	GC(time.Duration)
-}
-
-type (
-	// Provider implements the IProvider
-	// contains the temp sessions memory, the store and some options for the cookies
-	Provider struct {
-		name               string
-		mu                 sync.Mutex
-		sessions           map[string]*list.Element // underline TEMPORARY memory store
-		list               *list.List               // for GC
-		NewStore           func(sessionId string, cookieLifeDuration time.Duration) store.IStore
-		OnDestroy          func(store store.IStore) // this is called when .Destroy
-		cookieLifeDuration time.Duration
-	}
-)
-
-var _ IProvider = &Provider{}
-
-// NewProvider returns a new empty Provider
-func NewProvider(name string) *Provider {
-	provider := &Provider{name: name, list: list.New()}
-	provider.sessions = make(map[string]*list.Element, 0)
-	return provider
-}
-
-// Init creates the store for the first time for this session and returns it
-func (p *Provider) Init(sid string) (store.IStore, error) {
-	p.mu.Lock()
-
-	newSessionStore := p.NewStore(sid, p.cookieLifeDuration)
-
-	elem := p.list.PushBack(newSessionStore)
-	p.sessions[sid] = elem
-	p.mu.Unlock()
-	return newSessionStore, nil
-}
-
-// Read returns the store which sid parameter is belongs
-func (p *Provider) Read(sid string) (store.IStore, error) {
-	p.mu.Lock()
-	if elem, found := p.sessions[sid]; found {
-		p.mu.Unlock() // yes defer is slow
-		return elem.Value.(store.IStore), nil
-	}
-	p.mu.Unlock()
-	// if not found
-	sessionStore, err := p.Init(sid)
-	return sessionStore, err
-
-}
-
-// Destroy always returns a nil error, for now.
-func (p *Provider) Destroy(sid string) error {
-	p.mu.Lock()
-	if elem, found := p.sessions[sid]; found {
-		elem.Value.(store.IStore).Destroy()
-		delete(p.sessions, sid)
-		p.list.Remove(elem)
-	}
-	p.mu.Unlock()
-	return nil
-}
-
-// Update updates the lastAccessedTime, and moves the memory place element to the front
-// always returns a nil error, for now
-func (p *Provider) Update(sid string) error {
-	p.mu.Lock()
-
-	if elem, found := p.sessions[sid]; found {
-		elem.Value.(store.IStore).SetLastAccessedTime(time.Now())
-		p.list.MoveToFront(elem)
-	}
-
-	p.mu.Unlock()
-	return nil
-}
-
-// GC clears the memory
-func (p *Provider) GC(duration time.Duration) {
-	p.mu.Lock()
-	p.cookieLifeDuration = duration
-	defer p.mu.Unlock() //let's defer it and trust the go
-
-	for {
-		elem := p.list.Back()
-		if elem == nil {
-			break
-		}
-
-		// if the time has passed. session was expired, then delete the session and its memory place
-		if (elem.Value.(store.IStore).LastAccessedTime().Unix() + duration.Nanoseconds()) < time.Now().Unix() {
-			p.list.Remove(elem)
-			delete(p.sessions, elem.Value.(store.IStore).ID())
-
-		} else {
-			break
-		}
-	}
-}
-
-// Name the provider's name, example: 'memory' or 'redis'
-func (p *Provider) Name() string {
-	return p.name
-}