Summary:
This diff introduce `IOverlay` that provides a shared interface between
`FSOverlay` and `SqliteOverlay`. This allows us to share more code between
POSIX and Windows, and prepare for the new SQLite-backed Overlay
implementation.
Note as of this diff `IOverlay` is only at a very initial version. The
interface is far from complete and still need a few iteration to change.
Reviewed By: chadaustin
Differential Revision: D23941454
fbshipit-source-id: 69cce9d3852f21b2d19f528e7e05fb183538ace9