protect raylet against bad messages (#4003)

* protect raylet against bad messages

* address comments

* linting and regression test

python/ray/includes/ray_config.pxd

@@ -8,7 +8,7 @@ cdef extern from "ray/ray_config.h" nogil:
         @staticmethod
         RayConfig &instance()
 
-        int64_t ray_protocol_version() const
+        int64_t ray_cookie() const
 
         int64_t handler_warning_timeout_ms() const
 

python/ray/includes/ray_config.pxi

@@ -2,8 +2,8 @@ from ray.includes.ray_config cimport RayConfig
 
 cdef class Config:
     @staticmethod
-    def ray_protocol_version():
-        return RayConfig.instance().ray_protocol_version()
+    def ray_cookie():
+        return RayConfig.instance().ray_cookie()
 
     @staticmethod
     def handler_warning_timeout_ms():