Maybe default value of table property write.upsert.enabled should be true

[baiyangtx]

Currently, the spark batch writes and the flink streaming write will use the same table property to guarantee the uniqueness of the primary key. And for spark writes, the uniqueness of the primary key should be guaranteed by default, on the other hand, I think the uniqueness of the primary key should be guaranteed at a high level, no matter whether in batch or streaming write so I think the default value for write.upsert.enabled should be true.

[baiyangtx]

@zhoujinsong @YesOrNo828 CC

[YesOrNo828]

Before changing the default value of the write.upsert.enabled to true, I think we should address this issue first #964 .