Fix resource management bug with replication=database.

 Fix resource management bug with replication=database.
 
 Commit 0d8c9c1210c44b36ec2efcb223a1dfbe897a3661 allowed BASE_BACKUP to
 acquire a ResourceOwner without a transaction so that the backup
 manifest functionality could use a BufFile, but it overlooked the fact
 that when a walsender is used with replication=database, it might have
 a transaction in progress, because in that mode, SQL and replication
 commands can be mixed.  Try to fix things up so that the two cleanup
 mechanisms don't conflict.
 
 Per buildfarm member serinus, which triggered the problem when
 CREATE_REPLICATION_SLOT failed from inside a transaction.  It passed
 on the subsequent run, so evidently the failure doesn't happen every
 time.