[Java] Allow actor handle to be serialized without forking

[jovany-wang]

What do these changes do?

1. fork() API will fork an actor handle explicitly that we could use it to do a other Ray.calls.
2. For serializer, we implicitly do fork when serializing recursively.

For example:

class MyObject {
private RayActor actor;
}

or

class MyObject {
private MyObject1 obj1;
}

class MyObject1 {
private RayActor actor;
}

If the actor object actor is a field of another object obj(MyObject), when we pass obj by Ray.call, the obj.actor's handleId will be changed to another id.

Related issue number

N/A

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