pgsql: Postpone some more stuff out of ExecInitModifyTable.

Postpone some more stuff out of ExecInitModifyTable.

Delay creation of the projections for INSERT and UPDATE tuples
until they're needed.  This saves a pretty fair amount of work
when only some of the partitions are actually touched.

The logic associated with identifying junk columns in UPDATE/DELETE
is moved to another loop, allowing removal of one loop over the
target relations; but it didn't actually change at all.

Extracted from a larger patch, which seemed to me to be too messy
to push in one commit.

Amit Langote, reviewed at different times by Heikki Linnakangas and
myself

Discussion: https://postgr.es/m/CA+HiwqG7ZruBmmih3wPsBZ4s0H2EhywrnXEduckY5Hr3fWzPWA@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/a1115fa0782378a8238045d238ae70cac36be8ae

Modified Files
--------------
src/backend/executor/execMain.c        |   1 +
src/backend/executor/nodeModifyTable.c | 365 +++++++++++++++++++--------------
src/include/nodes/execnodes.h          |   2 +
3 files changed, 210 insertions(+), 158 deletions(-)