PSpace in thread local storage

The CSpace implementation managed to factor out an implementation that completely avoided thread local storage. Unfortunately that's not so easy with the PersistStore.

• IPersistable is fundamental to the underlying implementation
• IPersistable defines the method VisitPrefs
• IPrefVisitor directly visits pref members
• operator*() on a pref can only make sense if the associated PSpace is available in thread local storage

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PSpace* GetThreadPtr<PSpace>()

Get the pointer to the PSpace in thread local storage, or returns nullptr if none has been set.

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void SetThreadPtr(PSpace* pspace)

Set the pointer to the PSpace in thread local storage.

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PersistStore* GetThreadPersistStore()

If a PSpace is set in thread local storage then returns the associated PersistStore, otherwise returns nullptr.