Store BlobStorage pointer in ColumnFamilyHandle

[Connor1996]

rocksdb embed cfd pointer in ColumnFamilyHandleImpl. One of the benefit is, with the handle, a read doesn't need to acquire db_mutex to obtain cfd pointer. Similarly, we can embed BlobStorage pointer in the handle to save mutex lock to obtain BlobStorage pointer. We can define TitanColumnFamilyHandle as following and return it to caller. The struct is safe to be used as rocksdb::ColumnFamilyHandle when calling rocksdb methods.

struct TitanColumnFamilyHandle : public rocksdb::ColumnFamilyHandleImpl {
    std::shared_ptr<BlobStorage> blob_storage;
};

Part of #139