May remove blob storage too early

[Connor1996]

When calling DestroyColumnFamilyHandle we may remove related blob storage directly. But if there are multiples column family handles, Titan removes related blob storage on the first time to call DestroyColumnFamilyHandle, so using other non-destroyed column family handles to get will encounter blob-storage-not-found.

TitanDB::Open(db_options, dbname_, descs, &cf_handles_, &db_));

// ... put some data

std::vector<ColumnFamilyHandle*> cf_handles_tmp;
for (auto& handle : cf_handles_) {
    cf_handles_tmp.push_back(db_impl_->GetColumnFamilyHandleUnlocked(handle->GetID()));
 }

// ... get some data. Success!

for (auto& handle : cf_handles_tmp) {
    ASSERT_TRUE(handle);
    db_->DestroyColumnFamilyHandle(handle);
 }

// ... get some data. Fail!

But it's okay for normal usage, cause GetColumnFamilyHandleUnlocked can't be called exteriorly.