Summary: [Folly] Switch from `-defs.h` pattern to `-inl.h` pattern in `folly/stats/` since `extern template` doing shallow instantiation rather than deep instantiation makes this pattern difficult.

Reviewed By: chadaustin

Differential Revision: D16761319

fbshipit-source-id: 1760bf53b09791bebf996064c0d1fcc3babf43d2