Bridge Enhancement: Add a topic for the current remote broker address:port.

[cjmaynard]

Currently I can find out which remote broker the bridge is connected to by subscribing to "$SYS/broker/log/N", but because subscribing to this topic means I get all notices and not just the one I want, I have to parse through the data looking for messages of the form:

1504063227: Connecting bridge foo (1.2.3.4:1883)

It would be really nice if there was a specific topic that one could subscribe to in order to find out which remote broker the bridge is currently connected to. I suppose if there's no connection at all, then an empty address:port message could be delivered? Doing so would obviate the need for me to also subscribe to $SYS/broker/connection/+/state in order to find out if there's an active connection or not.

Thanks for reading.