Ref count objects created with ray.put (#5590)

* wip

* add weakref option

* tune util

* test ref

* doc

* fix tests

python/ray/includes/unique_ids.pxi

@@ -130,6 +130,7 @@ cdef class UniqueID(BaseID):
 
 cdef class ObjectID(BaseID):
     cdef CObjectID data
+    cdef object buffer_ref
 
     def __init__(self, id):
         check_id(id)
@@ -153,6 +154,12 @@ cdef class ObjectID(BaseID):
     def task_id(self):
         return TaskID(self.data.TaskId().Binary())
 
+    def set_buffer_ref(self, ref):
+        self.buffer_ref = ref
+
+    def get_buffer_ref(self):
+        return self.buffer_ref
+
     cdef size_t hash(self):
         return self.data.Hash()