pgsql: Refactor to reduce code duplication for function property checki

Refactor to reduce code duplication for function property checking.

As noted by Andres Freund, we'd accumulated quite a few similar functions
in clauses.c that examine all functions in an expression tree to see if
they satisfy some boolean test.  Reduce the duplication by inventing a
function check_functions_in_node() that applies a simple callback function
to each SQL function OID appearing in a given expression node.  This also
fixes some arguable oversights; for example, contain_mutable_functions()
did not check aggregate or window functions for mutability.  I doubt that
that represents a live bug at the moment, because we don't really consider
mutability for aggregates; but it might someday be one.

I chose to put check_functions_in_node() in nodeFuncs.c because it seemed
like other modules might wish to use it in future.  That in turn forced
moving set_opfuncid() et al into nodeFuncs.c, as the alternative was for
nodeFuncs.c to depend on optimizer/setrefs.c which didn't seem very clean.

In passing, teach contain_leaked_vars_walker() about a few more expression
node types it can safely look through, and improve the rather messy and
undercommented code in has_parallel_hazard_walker().

Discussion: <20160527185853.ziol2os2zskahl7v@alap3.anarazel.de>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/2f153ddfdd318b211590dd5585f65f357d85c2de

Modified Files
--------------
src/backend/nodes/nodeFuncs.c         | 178 +++++++++
src/backend/optimizer/plan/setrefs.c  |  63 ----
src/backend/optimizer/util/clauses.c  | 688 +++++++++-------------------------
src/backend/optimizer/util/predtest.c |   2 +-
src/backend/utils/cache/relcache.c    |   2 +-
src/include/nodes/nodeFuncs.h         |  10 +
src/include/optimizer/planmain.h      |   3 -
7 files changed, 371 insertions(+), 575 deletions(-)