Use std::thread in AtomicHashMapTest.cpp
Use std::thread in AtomicHashMapTest.cpp

Summary:
[Folly] Use `std::thread` in `AtomicHashMapTest.cpp` instead of using the pthread API.

In general in normal C++ code, prefer C++ standard library facilities over C facilities.

Reviewed By: Orvid

Differential Revision: D19226850

fbshipit-source-id: 7edc5fca25266b48ab3e8c8829708594d7563409

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