Refactor sessions and add locking to avoid concurrent requests to create

more instances

types/session.go

@@ -1,11 +1,14 @@
 package types
 
 import (
+	"sync"
+
 	"github.com/docker/docker/api/types"
 	"github.com/franela/play-with-docker/cookoo"
 )
 
 type Session struct {
+	sync.Mutex
 	Id        string               `json:"id"`
 	Instances map[string]*Instance `json:"instances"`
 }