Replace atomic var by normal var (#39)

Because db is locked when  add/sub  bg_gc_scheduled_, so this variable is no need to be atomic 

src/db_impl.h

@@ -239,7 +239,8 @@ class TitanDBImpl : public TitanDB {
   // pending_gc_ hold column families that already on gc_queue_.
   std::deque<uint32_t> gc_queue_;
 
-  std::atomic_int bg_gc_scheduled_{0};
+  // Guarded by mutex_.
+  int bg_gc_scheduled_{0};
 
   std::atomic_bool shuting_down_{false};
 };

src/db_impl_gc.cc

@@ -14,11 +14,9 @@ void TitanDBImpl::MaybeScheduleGC() {
 
   if (shuting_down_.load(std::memory_order_acquire)) return;
 
-  if (bg_gc_scheduled_.load(std::memory_order_acquire) >=
-      db_options_.max_background_gc)
-    return;
+  if (bg_gc_scheduled_ >= db_options_.max_background_gc) return;
 
-  bg_gc_scheduled_.fetch_add(1, std::memory_order_release);
+  bg_gc_scheduled_++;
 
   env_->Schedule(&TitanDBImpl::BGWorkGC, this, Env::Priority::BOTTOM, this);
 }
@@ -118,7 +116,7 @@ Status TitanDBImpl::BackgroundGC(LogBuffer* log_buffer) {
 Status TitanDBImpl::TEST_StartGC(uint32_t column_family_id) {
   {
     MutexLock l(&mutex_);
-    bg_gc_scheduled_.fetch_add(1, std::memory_order_release);
+    bg_gc_scheduled_++;
   }
   // BackgroundCallGC
   Status s;