Support removing and/or cleaning up sdo vouchers

[bmpotter]

These are all possibilities:

• Automatically remove the voucher when the corresponding node is removed from the exchange
• Support an explicit remove of a voucher
• Support a cleanup option that removes vouchers:
  • If their ocs/config/db/v1/devices/<device-id>/state.json file indicates that the device has been successfully sdo-booted
  • If the corresponding node no longer exists in the exchange