NewPlacementRuleManager sets and unset placement rules for online restore.

NewRestoreClient returns a new RestoreClient.

SortAndValidateFileRanges sort, merge and validate files by tables and yields tables with range.

If there are many tables with only a few rows, the number of merged SSTs will be too large.

RewriteModeKeyspace means the rewrite rule could be applied to keyspace.

RewriteModeLegacy means no rewrite rule is applied.

CreatedTable is a table created on restore process, but not yet filled with data.

PlacementRuleManager manages to set the placement rule of tables to label constraint key `exclusive`, and unset the rule.

RewriteMode is a mode flag that tells the TiKV how to handle the rewrite rules.