Using shard index to scatter integer hot index

[TonsnakeLin]

## Enhancement
Hot index reduces the write scalability of the TiDB cluster when the written data is monotonically increasing. Using an expression index to scatter the hot index, the new expression index is called a shard index. The expression prefix of shard index must be tidb_shard() that is a new TiDB built-in function.

In order to simplify the work, Only scatter integer unique index. e.g. there is a index "unique key(id)" and it is a hot index. Using the new index "unique key(tidb_shard(id), id)" to replace it.

There are scalability of hot index and shard index.
scalability of hot index

scalability of shard index