Support providing a Token to ReadPortal and WritePortal

When managing portals in the execution environment, ArrayHandle uses the Token object to ensure that the memory associated with a portal exists for the length of time that it is needed. This is done by creating the portal with a Token object, and the associated portal objects are guaranteed to be valid while that Token object exists. This is supported by essentially locking the array from further changes.

Token objects are typically used when creating a control-side portal with the ReadPortal or WritePortal. This is not to say that a Token would not be useful; a control-side portal going out of scope is definitely a problem. But the creation and distruction of portals in the control environment is generally too much work for the possible benefits.

However, under certain circumstances it could be useful to use a Token to get a control-side portal. For example, if the PrepareForExecution method of an ExecutionObjectBase needs to fill a small ArrayHandle on the control side to pass to the execution side, it would be better to use the provided Token object when doing so. This change allows you to optionally provide that Token when creating these control-side portals.

Fixes #723 (closed)