When use command `COPY FROM ON SEGMENT`, we copy data from
local file to the table on the segment directly. When copying
data, we need to apply the distribution policy on the record to compute
the target segment. If the target segment ID isn't equal to
current segment ID, we will report error to keep the distribution
key restriction.

Because the segment has no meta data info about table distribution policy and
partition policy,we copy the distribution policy of main table from
master to segment in the query plan. When the parent table and
partitioned sub table has different distribution policy, it is difficult
to check all the distribution key restriction in all sub tables. In this
case , we will report error.

In case of the partitioned table's distribution policy is
RANDOMLY and different from the parent table, user can use GUC value
`gp_enable_segment_copy_checking` to disable this check.

Check the distribution key restriction as follows:

1) Table isn't partioned:
    Compute the data target segment.If the data doesn't belong the
    segment, will report error.

2) Table is partitioned and the distribution policy of partitioned table
as same as the main table:
    Compute the data target segment.If the data doesn't belong
    the segment, will report error.

3) Table is partitioned and the distribution policy of partitioned
table is different from main table:
    Not support to check ,report error.
Signed-off-by: NXiaoran Wang <xiwang@pivotal.io>
Signed-off-by: NMing LI <mli@apache.org>
Signed-off-by: NAdam Lee <ali@pivotal.io>