[PATCH 5/8] Add a new syscache to fetch a pg_class entry via (reltablespace, relfilenode)

This patch is problematic because formally indexes used by syscaches needs to
be unique, this one is not though because of 0/InvalidOids relfilenode entries
for nailed/shared catalog entries. Those values cannot be sensibly queries from
the catalog anyway though (the relmapper infrastructure needs to be used).

It might be nicer to add infrastructure to do this properly, I just don't have
a clue what the best way for this would be.
---
 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(+)