Ensure Callable isolation for distributed executors

If Callable submitted for distributed execution contains mutable instance fields they might get mutated locally before being sent for a remote execution thus potentially resulting in wrong results being returned to a task invoker. Just before Callable is dispersed across cluster it gets invoked locally where instance fields of a submitted Callable can be possibly mutated; Callable is in turn sent to remote nodes with mutated values instead of "original" field values as submitted by user.

We need to ensure that each execution node gets a pristine version of Callable object submitted by the user.