pageserver: compaction "key not found" on RelMap keys error under some pg_regress edge case

[jcsp]

This is not seen in real systems, so we presume it is some edge case that pg_regress is exercising.

#8232 reproduces it reliably in debug mode on pg15.

Possible hypothesis: some inconsistency between collect_keyspace and database deletion: collect_keyspace is calling relmap_file_key for each database in the dbdir, but database deletion eliminates all such keys for a database.

[jcsp]

put_rel_creation has a branch where it inserts into dbdir if the (dbnode, spcnode) doesn't exist, but in that branch it doesn't call put_relmap_file like a normal database creation.

collect_keyspace assumes that if something is in dbdir then it must also have a relmap file -> boom.

[jcsp]

I was trying to hunt down which part of the regression tests creates this scenario: the offending relation is deleted by the time the tests end, so it's not easy to look it up in pg_class.

The weird thing about it is that it's using spcnode==16502, which is not one of the normal two spcnodes (1663, 1664)