the table names to store each kind of checkpoint in the checkpoint database remember to increase the version number in case of incompatible change.

shared by MySQLCheckpointsDB and GlueCheckpointsDB.

GlueCheckpointsDB is almost same with MySQLCheckpointsDB, but it uses TiDB's internal data structure which requires a lot to keep same with database/sql.

NullCheckpointsDB is a checkpoints database with no checkpoints.