Rename IncomingMessageEnvelopePicker to MessageChooser

The name for IncomingMessageEnvelopePicker is ugly, too long, and a bit weird. Not sure what I was thinking, but we should rename it.

I like MessageChooser. Short, and simple. It's a class that chooses messages.

This change should just be a simple class name refactor. No config changes are required since IncomingMessageEnvelopePicker is not configurable/pluggable right now.