Lightning support for placement rules

[morgo]

Lightning should be able to import a dumpling backup which has placement rules specified.

The default behavior is to ignore the placement rules, but in future we might add an option to import placement rules. This option will not be the default because lightning is a logical tool.

[lcwangchao]

We will a add new option to the lightning config file:

[lightning]
# "import": create associated placement policies in the target TiDB. If policies with the same name exist, ignore them.
# "ignore":  Ignore all policies in the source files. Default value.
tidb-placement-policy = "ignore" 

The behaviors of Lightning will be：

• placement-policy = "import"
  • Lightning will create schema with policy configs and associated policies. Ignore If policies already exist.
  • Other policies not associated will be ignored.
• placement-policy = "ignore" Default.
  • All policy configs and policies in the source files will be ignored.