Support direct execution migration of external worker service tasks

[basclaessen]

When a process instance with an active external worker service task is migrated to a new process definition with the same external worker service task the current external worker service task is removed and a new external worker service task is created.

When an external worker service task has been acquired by a worker but not completed before migration this will cause the following side effects:

1. the already acquired external worker service task can not be completed by the worker because this external worker service task has been deleted during migration
2. the external worker service task that has been created during migration can also be acquired by the worker and the work is executed twice

When support is added for direct execution migration of external worker service tasks (with the same configuration) the side effects mentioned above will not happen because the current execution will be migrated and the external worker service tasks will be updated in stead of deleted (current) and created (new).