Split up countable.h

Summary: Moved CountableHelper to object-data.h since it's only used for
ObjectData. Moved AtomicCountable to its own header file since it
really has nothing to do with normal countable objects. Removed most
includes from countable.h, fixing up cpp files where necessary.
Removed #define for STATIC and UNCOUNTABLE (const decls instead).
Cleaned up the refcount assertions.

Reviewed By: @jdelong

Differential Revision: D1753093

Signature: t1:1753093:1419107371:fc75bcb2e2385d4b16d5cbedbf2698dbf32d84db