release 3.0: rebase master (#253)
* diff: add connection manage (#237)
* column-mapping: set schemaID/tableID to 0 if numeric suffix is missing (#236)
* column-mapping: set schemaID/tableID to 0 if numeric suffix is missing
* column-mapping: demand an explicit separator
* pkg/column-mapping: addressed comments
* column-mapping: addressed comments
* diff: some minor fix (#242)
* remove row_id (#240)
* diff: update column's length for checkpoint table (#245)
* diff: add integration test (#239)
* diff: fix num is nil (#250)