pgsql: Add stack depth checks to key recursive functions inbackend/nod

Add stack depth checks to key recursive functions in backend/nodes/*.c.

Although copyfuncs.c has a check_stack_depth call in its recursion,
equalfuncs.c, outfuncs.c, and readfuncs.c lacked one.  This seems
unwise.

Likewise fix planstate_tree_walker(), in branches where that exists.

Discussion: https://postgr.es/m/30253.1544286631@sss.pgh.pa.us

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/62999b93251eefbfcf900a0bb202721b0a422ffd

Modified Files
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src/backend/nodes/equalfuncs.c | 4 ++++
src/backend/nodes/nodeFuncs.c  | 3 +++
src/backend/nodes/outfuncs.c   | 4 ++++
src/backend/nodes/readfuncs.c  | 4 ++++
4 files changed, 15 insertions(+)