[PATCH 04/14] Add a new RELFILENODE syscache to fetch a pg_class entry via (reltablespace, relfilenode)

This cache is somewhat problematic because formally indexes used by syscaches
needs to be unique, this one is not. This is "just" because of 0/InvalidOids
stored in pg_class.relfilenode for nailed/shared catalog relations.  The
syscache will never be queried for InvalidOid relfilenodes however so it seems
to be safe even if it violates the rules somewhat.

It might be nicer to add infrastructure to do this properly, like using a
partial index, its not clear what the best way to do this is though.

Needs a CATVERSION bump.
---
 src/backend/utils/cache/syscache.c | 11 +++++++++++
 src/include/catalog/indexing.h     |  2 ++
 src/include/catalog/pg_proc.h      |  1 +
 src/include/utils/syscache.h       |  1 +
 4 files changed, 15 insertions(+)