pgsql: Move partition_tuple_slot out of EState.

Move partition_tuple_slot out of EState.

Commit 2ac3ef7a01df859c62d0a02333b646d65eaec5ff added a TupleTapleSlot
for partition tuple slot to EState (es_partition_tuple_slot) but it's
more logical to have it as part of ModifyTableState
(mt_partition_tuple_slot) and CopyState (partition_tuple_slot).

Discussion: http://postgr.es/m/1bd459d9-4c0c-197a-346e-e5e59e217d97@lab.ntt.co.jp

Amit Langote, per a gripe from me

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/345b2dcf070bd8fbccde643b1b2856027623e9e5

Modified Files
--------------
src/backend/commands/copy.c | 26 +++++++++++++-------------
src/backend/executor/execMain.c | 12 ++++++++++++
src/backend/executor/nodeModifyTable.c | 17 ++++++++---------
src/include/executor/executor.h | 1 +
src/include/nodes/execnodes.h | 4 +---
5 files changed, 35 insertions(+), 25 deletions(-)