Summary:
Most stat code in fbcode was moved in to open source under
fbcode/fb303. The only exception that we use internally is the
MonotonicCounter class. This diff moves all of our stats code to use
the open source headers and updates cmake files to pull in the open
source fb303.

Reviewed By: chadaustin, shri-khare

Differential Revision: D16969960

fbshipit-source-id: 12b7abb54c956c242c8e27eb69fd96925c7e61f7