Don't include execnodes.h in replication/conflict.h

Don't include execnodes.h in replication/conflict.h

... which silently propagates a lot of headers into many places
via pgstat.h, as evidenced by the variety of headers that this patch
needs to add to seemingly random places.  Add a minimum of typedefs to
conflict.h to be able to remove execnodes.h, and fix the fallout.

Backpatch to 18, where conflict.h first appeared.

Discussion: https://postgr.es/m/202509191927.uj2ijwmho7nv@alvherre.pgsql