tables in the query compete for cache space, not just the one we are
currently costing an indexscan for.  This seems more realistic, and it
definitely will help in examples recently exhibited by Stefan
Kaltenbrunner.  To get the total size of all the tables involved, we must
tweak the handling of 'append relations' a bit --- formerly we looked up
information about the child tables on-the-fly during set_append_rel_pathlist,
but it needs to be done before we start doing any cost estimation, so
push it into the add_base_rels_to_query scan.