Pick the destination for a given message.
Pick the destination for a given message. Normally it picks one of the
routees, but in the end it is up to the implementation to
return whatever Routee to use for sending a specific message.
When implemented from Java it can be good to know that
routees.apply(index) can be used to get an element
Uses round-robin to select a routee. For concurrent calls, round robin is just a best effort.