UnsafeRow is an InternalRow that is backed by raw memory instead of Java objects.
UnSafeRow has three parts: [null bit set] [values] [variable length portion]
- 64bit（8byte）对齐，内存空间不紧凑但有利于提高访存性能
- 小端存储，这样低位类型存到高位内存（如存int到64位）不需要额外编码
- 所有列不管什么类型都按64bit存储，变长内容顺延存储
