Make RelationFlushRelation() work without ResourceOwner during abort

ReorderBufferImmediateInvalidation() executes invalidation messages in
an aborted transaction. However, RelationFlushRelation sometimes
required a valid resource owner, to temporarily increment the refcount
of the relache entry. Commit b8bff07daa worked around that in the main
subtransaction abort function, AbortSubTransaction(), but missed this
similar case in ReorderBufferImmediateInvalidation().

To fix, introduce a separate function to invalidate a relcache
entry. It does the same thing as RelationClearRelation(rebuild==true)
does when outside a transaction, but can be called without
incrementing the refcount.

Add regression test. Before this fix, it failed with:

ERROR: ResourceOwnerEnlarge called after release started

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://www.postgresql.org/message-id/e56be7d9-14b1-664d-0bfc-00ce9772721c@gmail.com

src/backend/access/transam/xact.c

@@ -5279,20 +5279,7 @@ AbortSubTransaction(void)
 
 		AtEOSubXact_RelationCache(false, s->subTransactionId,
 								  s->parent->subTransactionId);
-
-
-		/*
-		 * AtEOSubXact_Inval sometimes needs to temporarily bump the refcount
-		 * on the relcache entries that it processes.  We cannot use the
-		 * subtransaction's resource owner anymore, because we've already
-		 * started releasing it.  But we can use the parent resource owner.
-		 */
-		CurrentResourceOwner = s->parent->curTransactionOwner;
-
 		AtEOSubXact_Inval(false);
-
-		CurrentResourceOwner = s->curTransactionOwner;
-
 		ResourceOwnerRelease(s->curTransactionOwner,
 							 RESOURCE_RELEASE_LOCKS,
 							 false, false);