message: parallelize destruction
  Summary:
  Destruction if concurrent containers tends to happen on the main thread, and can be surprisingly expensive. Parallelizing it speeds things up.
  This is a behavior-preserving change.
diff: #include "Debug.h"
  #include "Timer.h"
  +#include "WorkQueue.h"
  // Forward declaration.
  namespace cc_impl {
  @@ -62,9 +63,14 @@ class ConcurrentContainer {
  virtual ~ConcurrentContainer() {
  auto timer_scope = cc_impl::s_destructor.scope();
  + if (size() < 4096) {
  for (size_t slot = 0; slot < n_slots; ++slot) {
  m_slots[slot] = Container();
  }
  + return;
  + }
  + workqueue_run_for<size_t>(
  + 0, n_slots, [this](size_t slot) { m_slots[slot] = Container(); });
  }
  /*
  