Fix relcache reference leak when decoding TRUNCATE

ReorderBufferProcessTXN() opens every relation referenced by a
TRUNCATE change.  When RelationIsLogicallyLogged() returns false,
it skips the relation without releasing the reference acquired
by RelationIdGetRelation().

Looking at the in-core code paths building XLOG_HEAP_TRUNCATE records,
no relation OIDs would be included if they do not satisfy
RelationIsLogicallyLogged().  One pattern that could go through is if a
table is switched to SET UNLOGGED, but that would not be reachable in
practice as the decoding happens after a historical snapshot is taken,
so the relation should still be valid.

This is a defense-in-depth measure in practice, and we tend to be
careful about how Relations are handled when sending changes to output
plugins, so backpatch all the way down.

Author: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/7DD65D03-3B5A-43B2-99AD-8E6AF5372BAB@gmail.com
Backpatch-through: 14

src/backend/replication/logical/reorderbuffer.c

@@ -2376,7 +2376,10 @@ ReorderBufferProcessTXN(ReorderBuffer *rb, ReorderBufferTXN *txn,
 								elog(ERROR, "could not open relation with OID %u", relid);
 
 							if (!RelationIsLogicallyLogged(rel))
+							{
+								RelationClose(rel);
 								continue;
+							}
 
 							relations[nrelations++] = rel;
 						}