Deleting a switch port settings object can cause dangling references

[internet-diglett]

Currently the switch port configuration process is as follows:

1. an operator defines a desired configuration via switch-port-settings
2. the operator then applies the defined configuration via switch-port-settings apply

However, if an operator decides to delete a defined configuration object, we do not currently check to see if it is "in use" or "applied" to any switch ports. This means you can delete the configuration and the RPWs will then attempt to apply a nonexistent configuration to the switch port.

To resolve this bug, we should probably update the transaction to check to see if a configuration is in use before deleting it.