Include network four-tuple for connections in io.grpc.ChannelLogger logs

We can't include them in each log message, because we simply don't know the values much of the time. But specifically for the transportReady and transportShutdown notifications, it seems we can plumb it without synchronization. We can add methods to ManagedClientTransport to get the local/remote addresses, maybe just as a `getAttributes()` method.

There's probably some status messages that would also be useful to include the four-tuple, but it may be hard to track them. Maybe we could include it in the DEADLINE_EXCEEDED message we generate.