Split getWrapperTL from getWrapper in SingletonThreadLocal
Split getWrapperTL from getWrapper in SingletonThreadLocal

Summary:
[Folly] Split `getWrapperTL` from `getWrapper` in `SingletonThreadLocal`.

Allows hypothetical future changes to operate on the `ThreadLocal<Wrapper>` singleton itself, in addition to the existing code which operates on the `Wrapper` per-thread-singleton.

Reviewed By: djwatson

Differential Revision: D7316099

fbshipit-source-id: 80a8b7ce28da1bc64b204292032766c648f15944

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