Future can't be passed to actor method

[YarShev]

What happened:

Future is not materialized in an actor method by Dask machinery.

What you expected to happen:

Future is able to be materialized in an actor method by Dask machinery how it is done to submit ordinary functions.

Minimal Complete Verifiable Example:

from distributed.client import Client

client = Client()

class Foo:
    def __init__(self):
        self._accumulator = 0
    def foo(self, x):
        self._accumulator += x

f1 = client.submit(Foo, actor=True)
actor = f1.result()
f2 = client.scatter(7)
f3 = actor.foo(f2)
TypeError: unsupported operand type(s) for +=: 'int' and 'Future'

Anything else we need to know?:

Related to #4488.

Environment:

• Dask version: 2.19.0
• Python version: Python 3.8.8
• Operating System: Ubuntu 20.04.1 LTS
• Install method (conda, pip, source): pip