Fix failure to delete spill files of aborted transactions

Logical decoding's reorderbuffer.c may spill transaction files to disk
when transactions are large.  These are supposed to be removed when they
become "too old" by xid; but file removal requires the boundary LSNs of
the transaction to be known.  The final_lsn is only set when we see the
commit or abort record for the transaction, but nothing sets the value
for transactions that crash, so the removal code misbehaves -- in
assertion-enabled builds, it crashes by a failed assertion.

To fix, modify the final_lsn of transactions that don't have a value
set, to the LSN of the very latest change in the transaction.  This
causes the spilled files to be removed appropriately.

Author: Atsushi Torikoshi
Reviewed-by: Kyotaro HORIGUCHI, Craig Ringer, Masahiko Sawada
Discussion: https://postgr.es/m/54e4e488-186b-a056-6628-50628e4e4ebc@lab.ntt.co.jp

src/include/replication/reorderbuffer.h

@@ -168,6 +168,8 @@ typedef struct ReorderBufferTXN
 	 * * plain abort record
 	 * * prepared transaction abort
 	 * * error during decoding
+	 * * for a crashed transaction, the LSN of the last change, regardless of
+	 *   what it was.
 	 * ----
 	 */
 	XLogRecPtr	final_lsn;