"Collapsing" router or contrib pattern

This is a nice pattern we could implement as either a router or example. Tomek Nurkiewicz recently blogged about how Hystrix has this built in here: http://www.nurkiewicz.com/2014/11/batching-collapsing-requests-in-hystrix.html

The pattern is about lessening the number of requests sent to an "expensive to call" resource by "collapsing" multiple calls within some time into one message to that expensive resource and then responding back to all interested parties. We can easily implement something like this, so I figured it would be a nice task to pick up if someone wants to contribute a "specialised single destination collapsing Router" like this :-)

Hystrix docs: https://github.com/Netflix/Hystrix/wiki/How-To-Use#Collapsing