pgsql: Avoid leaking memory while evaluating arguments for a table func

Avoid leaking memory while evaluating arguments for a table function.

ExecMakeTableFunctionResult evaluated the arguments for a function-in-FROM
in the query-lifespan memory context.  This is insignificant in simple
cases where the function relation is scanned only once; but if the function
is in a sub-SELECT or is on the inside of a nested loop, any memory
consumed during argument evaluation can add up quickly.  (The potential for
trouble here had been foreseen long ago, per existing comments; but we'd
not previously seen a complaint from the field about it.)  To fix, create
an additional temporary context just for this purpose.

Per an example from MauMau.  Back-patch to all active branches.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d297c91d43798d19b80d5465349bb0f53fc9499a

Modified Files
--------------
src/backend/executor/execQual.c         |   15 +++++++++++----
src/backend/executor/nodeFunctionscan.c |   15 +++++++++++++++
src/include/executor/executor.h         |    1 +
src/include/nodes/execnodes.h           |    2 ++
4 files changed, 29 insertions(+), 4 deletions(-)