Add support for concurrent consumers on ftp endpoints

For example have a pool of worker threads to download the files in parallel.
Or implement a ConsumerPool (like we have for ProducerPool) and have ways to configure the pool settings.

Then you can have concurrent consumers on the ftp endpoint. The drawback is that they all would race for the same files. And thus you would need to ensure the
read lock strategy works, so multiple consumers do not download the same files.

See user forum
http://fusesource.com/forums/thread.jspa?messageID=12061&#12061