See line 227 in `CEFB` - he throws an exception if we put a poller in XML when wiring to a subscribable channel. We should probably be consistent and throw an exception here (see the tests I pushed to my branch).

On the other hand, given that the config is in java, perhaps there's some value in just ignoring the poller if the endpoint is wired to a subcribable channel. 

I wonder what @markfisher thinks.