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
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/552c310ba4ec072a2df48a08722f0af2e3703fdf

Modified Files
--------------
src/backend/nodes/equalfuncs.c | 4 ++++
src/backend/nodes/outfuncs.c   | 4 ++++
src/backend/nodes/readfuncs.c  | 4 ++++
3 files changed, 12 insertions(+)