Log Message:
Changes 9.1.5:
* Prevent access to external files/URLs via XML entity references
* Prevent access to external files/URLs via contrib/xml2's xslt_process()
* Prevent too-early recycling of btree index pages
* Fix crash-safety bug with newly-created-or-reset sequences
* Fix race condition in enum-type value comparisons
* Fix txid_current() to report the correct epoch when not in hot standby
* Prevent selection of unsuitable replication connections as the synchronous
  standby
* Fix bug in startup of Hot Standby when a master transaction has many
  subtransactions
* Ensure the backup_label file is fsync'd after pg_start_backup()
* Fix timeout handling in walsender processes
* Wake walsenders after each background flush by walwriter
* Fix LISTEN/NOTIFY to cope better with I/O problems, such as out of disk space
* Only allow autovacuum to be auto-canceled by a directly blocked process
* Improve logging of autovacuum cancels
* Fix WITH attached to a nested set operation (UNION/INTERSECT/EXCEPT)