Move supervisor to it's own package

It allows to keep main namespace cleaner

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

supervisor/create.go

@@ -0,0 +1,31 @@
+package supervisor
+
+type StartEvent struct {
+	s *Supervisor
+}
+
+func (h *StartEvent) Handle(e *Event) error {
+	container, io, err := h.s.runtime.Create(e.ID, e.BundlePath, e.Console)
+	if err != nil {
+		return err
+	}
+	h.s.containerGroup.Add(1)
+	h.s.containers[e.ID] = &containerInfo{
+		container: container,
+	}
+	ContainersCounter.Inc(1)
+	task := &StartTask{
+		Err:           e.Err,
+		IO:            io,
+		Container:     container,
+		Stdin:         e.Stdin,
+		Stdout:        e.Stdout,
+		Stderr:        e.Stderr,
+		StartResponse: e.StartResponse,
+	}
+	if e.Checkpoint != nil {
+		task.Checkpoint = e.Checkpoint.Name
+	}
+	h.s.tasks <- task
+	return errDeferedResponse
+}