fixed bugs
  1) in the windows query results from grouping by a nchar column could be inconsistent with that from taos cliEnt shell in the linux
  2) query of last record could lead to server crash
  3) both inserting data and creating dropped tables would fail if setting 'tables' option to 1

enhancements
  ) avoid repetitively parsing the same table when inserting records into multiple tables in a batch
  2) accelerate the speed of writing into a database with multiple replicas by redirecting the request to the master vnode
  3) clarify the error message when importing multiple files