Support COPY statement that imports the data file on segments directly
parallel. It could be used to import data files generated by "COPY ...
to ... ON SEGMENT'.

This commit also supports all kinds of data file formats which "COPY ...
TO" supports, processes reject limit numbers and logs errors accordingly.

Key workflow:
   a) For COPY FROM, nothing changed by this commit, dispatch modified
   COPY command to segments at first, then read data file on master, and
   dispatch the data to relevant segment to process.

   b) For COPY FROM ON SEGMENT, on QD, read dummy data file, other parts
   keep unchanged, on QE, process the data stream (empty) dispatched
   from QD at first, then re-do the same workflow to read and process
   the local segment data file.
Signed-off-by: NMing LI <mli@pivotal.io>
Signed-off-by: NAdam Lee <ali@pivotal.io>
Signed-off-by: NHaozhou Wang <hawang@pivotal.io>
Signed-off-by: NXiaoran Wang <xiwang@pivotal.io>