catalog checkpoint: one Batch represents a table, such as : mo_databases, mo_tables, mo_columns,..

CatalogCkp contains information about database and tables in the system,and MetadataCkp contains information about blocks.

Int64Map mainly used in unit test.

metadata checkpoint: Batch is a batch of block metadata for a table, one row of Batch represents a block meta data.

How to parse and handle PrecommiWriteCmd , pls ref to tae/rpc/handle.go/HandlePreCommit function.

CN pull the log tail of table from TN.

TNPingRequest ping request.

TNPingResponse ping response.

TNStringResponse string type response.