Added add,remove and get methods for targetConnectionFactories in

AbstractRoutingConnectionFactory

Added protected methods for
addTargetConnectionFactory,removeTargetConnectionFactory and
getTargetConnectionFactory so clients manipulate the map if needed

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Issue: AMQP-405
Thanks @artembilan for the comments.

I suppose it makes sense to add all the `ConnctionListener`s to the added `ConnectionFactory`. I'll make that change for now and you can let me know if you want it done differently.