Fix off-by-one in logical slot resource retention

When Who What
2017-10-01 21:35:49 Daniel Gustafsson (d_gustafsson) Closed in commitfest 2017-09 with status: Returned with feedback
2017-09-01 12:28:02 Aleksander Alekseev (a.alekseev) Posted review with messageid <20170901122802.18926.94043.pgcf@coridan.postgresql.org>
2017-09-01 12:28:02 Aleksander Alekseev (a.alekseev) New status: Waiting on Author
2017-09-01 12:22:30 Aleksander Alekseev (a.alekseev) Added a.alekseev as reviewer
2017-03-09 03:19:40 Craig Ringer (ringerc) Changed gitlink to https://github.com/2ndQuadrant/postgres/tree/dev/confirmed_flush_offbyone_fix
2017-03-09 03:19:10 Craig Ringer (ringerc) Changed authors to Craig Ringer (ringerc)
2017-03-09 03:19:01 Craig Ringer (ringerc) Attached mail thread CAMsr+YGFvikx-U_mHQ0mAzTarqvCpwzvsPKv=7MfP9scDrMPjg@mail.gmail.com
2017-03-09 03:19:01 Craig Ringer (ringerc) Created patch record