Controller account must be alive

[shawntabrizi]

Closes #6415

This PR updates the logic in the Staking Pallet to require that the controller account is alive and stays alive. This requirement already implicitly exists for stash accounts.

To do this, we introduce a check on the controller account in bond and set_controller to make sure the the account is not is_dead_account(). After passing this check, we place a reference counter on the controller, forcing it to stay alive as long as it stays a controller.

To make sure the account can later be killed, we remove the reference counter from old controllers in set_controller and kill_stash.

Tests have been added and updated to verify this behavior.

[shawntabrizi]

Closing and will open another pr which allows for creating deposit for controllers