External Data Source: Thrift structure changes

A few changes to the external data source thrift types:
* Change RowBatch to return entire columns. Adds Data.TColumnData to
  represent an entire column.
* Makes all fields in ExternalDataSource (except for status fields on
  the result structures) optional in case fields become deprecated in
  the future.
* Adds a limit parameter to the TOpenParams structure in case the
  data source needs to apply the limit itself.

Change-Id: I62db68bfb64d2190dfdd0c84be5925ad5db031ef
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2345
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins