dataMgr:importer/whImporter: add support to append records

1) Add support to append records from a datafile to a datatable.
2) Rename getRecordField() to getRecordFieldSepBySpace(), which means
   it only works for the records separated by space.
3) Add getRecordFieldSepByComma() which works for the records separated
   by comma.
4) Rename getRecordTime() and getRecordData() to getDirRecordTime() and
   getDirRecordData() because they should only be used with the records
   in a directory.

Signed-off-by: Zhengwang Ruan <ruan.zhengwang@gmail.com>