Return value of getSchedulers is not safe

[rleungx]

In server/cluster/coordinator.go, we have a function named getSchedulers.

func (c *coordinator) getSchedulers() map[string]*scheduleController {
	c.RLock()
	defer c.RUnlock()
        return c.schedulers
}

But here we directly return a map that may not safe if we do some operations concurrently on this map outside.