This patch modifies the way copy data is shared in order to mitigate potential race conditions
and ensure that the serialised controller stored in the DB is always in a valid state.

The restore controller is now considered the "source of truth" for all information about the
copy operation. Backup controllers can no longer contain information about course copies.

As copy creation is not atomic, it is still possible for copy controllers to become orphaned or
exist in an invalid state. To mitigate this the backup cleanup task has been modified to call
a new helper method copy_helper::cleanup_orphaned_copy_controllers.

Summary of changes in this patch:

    - Copy data must now be passed through the restore controller's constructor
    - base_controller::get_copy has been deprecated in favour of restore_controller::get_copy
    - base_controller::set_copy has been deprecated without replacement
    - core_backup\copy\copy has been deprecated, use copy_helper.class.p...