message: synchronize the ThreadLocalStats registry with SharedMutex instead of SpinLock
  Summary:
  SpinLocks behave poorly under contention and preemption, and one
  probably wasn't intended here. Switch the stats registry to
  synchronize with SharedMutex.
diff: #include <thread>
  #include <folly/SharedMutex.h>
  -#include <folly/SpinLock.h>
  #include <folly/synchronization/SmallLocks.h>
  #include <fb303/ThreadLocalStats.h>
  @@ -200,11 +199,7 @@ class TLStatsThreadSafeT {
  using StatGuard = std::lock_guard<const ContainerAndLock>;
  - /*
  - * The registry in the main ThreadLocalStatsT object is protected with a
  - * spinlock.
  - */
  - using RegistryLock = folly::SpinLock;
  + using RegistryLock = folly::SharedMutex;
  /**
  * The type to use for integer counter values.
  