Summary:
This adds a common/ directory with stub files required for building
eden. These stubs are the same ones as used in the fboss repository:
https://github.com/facebook/fboss/
Our goal is to eventually create an opensource fb303 repository with
complete implementations of these files, but for now these basic stubs
allow building eden. (Most of the underlying stats code that supports
fb303 is already available in folly/stats/ in the folly repository.)
Test Plan:
Tested building eden on Ubuntu 16.04
