Alerting on fdb count

I needed alerts when switch fdb counts got to predefined levels. I added the port fdb counts not because I needed them but because I saw someone else ask about them, and I was in there.

The problem here is that really, the fdb count should be associated with the device or port, but the code as structured needs all of the checks for a device or a port to be performed at once. So, to get this working, I created "device_fdb_count" and "port_fdb_count" entity types. I leave it to the developers to decide if this is an OK solution, or if there's a better way to collect values to use in the checkers.