Make PosixLogger::flush_pending_ atomic

Summary:
TSAN sometimes complaints data race of PosixLogger::flush_pending_. Make it atomic.
Closes https://github.com/facebook/rocksdb/pull/2231

Differential Revision: D4973397

Pulled By: siying

fbshipit-source-id: 571e886e3eca3231705919d573e250c1c1ec3764

env/posix_logger.h

@@ -41,7 +41,7 @@ class PosixLogger : public Logger {
   const static uint64_t flush_every_seconds_ = 5;
   std::atomic_uint_fast64_t last_flush_micros_;
   Env* env_;
-  bool flush_pending_;
+  std::atomic<bool> flush_pending_;
  public:
   PosixLogger(FILE* f, uint64_t (*gettid)(), Env* env,
               const InfoLogLevel log_level = InfoLogLevel::ERROR_LEVEL)