Summary:
FanoutSender is like FanoutChannel, except that instead of listening to and fanning out values from an input receiver, it allows values to be directly pushed into the sender.

This diff changes FanoutChannel to use FanoutSender, increasing code re-use. FanoutChannel now listens to values from the input receiver, and pushes them into a FanoutSender.

Reviewed By: aary

Differential Revision: D30889891

fbshipit-source-id: 6d2ae416a5a0a895a1b1269d21f6830d45d92184