message: Back out "re-introduce boost thread to sparta work queue"
  Summary: After committing diff (https://github.com/facebook/redex/commit/00ee212fd41af7c18a0874618cfe59a7164c9a5a), there is no need to use Boost threads anymore. The Sparta team has requaested we remove the Boost dependency from their code.
diff: #include <algorithm>
  #include <atomic>
  #include <boost/optional/optional.hpp>
  -#include <boost/thread/thread.hpp>
  #include <cassert>
  #include <chrono>
  #include <condition_variable>
  #include <numeric>
  #include <queue>
  #include <random>
  +#include <thread>
  #include <utility>
  #include "Arity.h"
  @@ -30,7 +30,7 @@ namespace parallel {
  * Sparta uses the number of physical cores.
  */
  static inline unsigned int default_num_threads() {
  - unsigned int threads = boost::thread::physical_concurrency();
  + unsigned int threads = std::thread::hardware_concurrency();
  return std::max(1u, threads);
  }
  @@ -304,13 +304,10 @@ void SpartaWorkQueue<Input, Executor>::run_all() {
  }
  }
  - std::vector<boost::thread> all_threads;
  + std::vector<std::thread> all_threads;
  all_threads.reserve(m_num_threads);
  for (size_t i = 0; i < m_num_threads; ++i) {
  - boost::thread::attributes attrs;
  - attrs.set_stack_size(8 * 1024 * 1024);
  - all_threads.emplace_back(attrs,
  - std::bind<void>(worker, m_states[i].get(), i));
  + all_threads.emplace_back(std::bind<void>(worker, m_states[i].get(), i));
  }
  for (auto& thread : all_threads) {
  