fsync pg_logical/mappings in CheckPointLogicalRewriteHeap().

While individual logical rewrite files were synced to disk, the directory was
not. On some filesystems that could lead to loosing directory entries after a
crash.

Reported-By: Tom Lane <tgl@sss.pgh.pa.us>
Author: Nathan Bossart <bossartn@amazon.com>
Discussion: https://postgr.es/m/867F2E29-2782-4869-970E-B984C6D35A8F@amazon.com
Backpatch: 10-

src/backend/access/heap/rewriteheap.c

@@ -1282,4 +1282,7 @@ CheckPointLogicalRewriteHeap(void)
 		}
 	}
 	FreeDir(mappings_dir);
+
+	/* persist directory entries to disk */
+	fsync_fname("pg_logical/mappings", true);
 }