Add new cli command to point the switch to a different backend

[HarrisKirk]

bgdctl switch backend

The above will route traffic to the designated backend IP. If the IP is not on the list of backend IPs, an exception is thrown

[chrissound]

This is now completed and ready for testing/merge #106

[HarrisKirk]

I left some comments in the PR. We can discuss over the phone.