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

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