Fix DBImpl::GetColumnFamilyHandleUnlocked data race (facebook#4666)

Summary:
Hi, yiwu-arbug, I found that `DBImpl::GetColumnFamilyHandleUnlocked` still have data race condition, because `column_family_memtables_` has a stateful cache `current_` and `column_family_memtables_::Seek` maybe call without the protection of `mutex_` by a write thread

check https://github.com/facebook/rocksdb/blob/859dbda6e3cac17416aff48f1760d01707867351/db/write_batch.cc#L1188  and   https://github.com/facebook/rocksdb/blob/859dbda6e3cac17416aff48f1760d01707867351/db/write_batch.cc#L1756  and  https://github.com/facebook/rocksdb/blob/859dbda6e3cac17416aff48f1760d01707867351/db/db_impl_write.cc#L318

So it's better to use `versions_->GetColumnFamilySet()->GetColumnFamily` instead.
Pull Request resolved: facebook#4666

Differential Revision: D13027117

Pulled By: yiwu-arbug

fbshipit-source-id: 4e3778eaf8e7f7c8577bbd78129b6a5fd7ce79fb
(cherry picked from commit 09426ae)

db/db_impl.cc

@@ -2056,16 +2056,16 @@ ColumnFamilyHandle* DBImpl::GetColumnFamilyHandle(uint32_t column_family_id) {
 // REQUIRED: mutex is NOT held.
 std::unique_ptr<ColumnFamilyHandle> DBImpl::GetColumnFamilyHandleUnlocked(
     uint32_t column_family_id) {
-  ColumnFamilyMemTables* cf_memtables = column_family_memtables_.get();
-
   InstrumentedMutexLock l(&mutex_);
 
-  if (!cf_memtables->Seek(column_family_id)) {
+  auto* cfd =
+      versions_->GetColumnFamilySet()->GetColumnFamily(column_family_id);
+  if (cfd == nullptr) {
     return nullptr;
   }
 
   return std::unique_ptr<ColumnFamilyHandleImpl>(
-      new ColumnFamilyHandleImpl(cf_memtables->current(), this, &mutex_));
+      new ColumnFamilyHandleImpl(cfd, this, &mutex_));
 }
 
 void DBImpl::GetApproximateMemTableStats(ColumnFamilyHandle* column_family,