folly: fixes for use with `-fvisibility-inlines-hidden`

folly: fixes for use with `-fvisibility-inlines-hidden`

Summary:
- Annotate non-`const` static locals in inline functions which don't require
  a single copy be used globally at runtime.
- Move implmentation from header file to source file (to avoid multiple
  copies at runtime).
- Mark a non-stateful static local as `const` to it easy to ignore when
  searching for problematic static locals.

Reviewed By: yfeldblum

Differential Revision: D4010101

fbshipit-source-id: 3be94a5dc5b7029a26e11b2145c0d41968979a5c