Fix drop column family data race

Summary:
A data race is caught by tsan_crash test between compaction and DropColumnFamily:
https://gist.github.com/yiwu-arbug/5a2b4baae05eeb99ae1719b650f30a44 Compaction checks if the column family has been dropped on each key input, while user can issue DropColumnFamily which updates cfd->dropped_, causing the data race. Fixing it by making cfd->dropped_ an atomic.
Closes https://github.com/facebook/rocksdb/pull/3250

Differential Revision: D6535991

Pulled By: yiwu-arbug

fbshipit-source-id: 5571df020beae7fa7db6fff5ad0d598f49962895

db/column_family.h

@@ -194,7 +194,7 @@ class ColumnFamilyData {
   // *) delete all memory associated with that column family
   // *) delete all the files associated with that column family
   void SetDropped();
-  bool IsDropped() const { return dropped_; }
+  bool IsDropped() const { return dropped_.load(std::memory_order_relaxed); }
 
   // thread-safe
   int NumberLevels() const { return ioptions_.num_levels; }
@@ -358,7 +358,7 @@ class ColumnFamilyData {
 
   std::atomic<int> refs_;      // outstanding references to ColumnFamilyData
   std::atomic<bool> initialized_;
-  bool dropped_;               // true if client dropped it
+  std::atomic<bool> dropped_;  // true if client dropped it
 
   const InternalKeyComparator internal_comparator_;
   std::vector<std::unique_ptr<IntTblPropCollectorFactory>>