CDC support for placement rules

[morgo]

For CDC it was determined that the placement rules will be replicated in the DDL stream, but as a comment (won't be applied on downstream systems). If in future a consumer of the CDC wants to make use of the placement rules it can do so, but because the CDC stream is logically, there is no current known common use case.

[lcwangchao]

To force placement will not be applied to the downstream, CDC should set tidb_placement_mode to ignore when the down stream is tidb.