Fix bug that can cause walsender not to terminating at shutdown.

Fix bug that can cause walsender not to terminating at shutdown.

When backpatching c6c333436 I (Andres Freund) mis-resolved a conflict
in the 9.4 branch. Unfortunately that leads to walsenders waiting
forever when shutting down with connected standbys, unless immediate
mode is used, or the standbys are forced to disconnect by other means.

Author: Marco Nenciarini
Reported-By: Abhijit Gharami, Marco Nenciarini
Discussion:
	http://postgr.es/m/1434ad79-ab0d-d6a9-4ea2-1afb1c06adaf@2ndquadrant.it
	http://postgr.es/m/CAA5_DuD0O1XyM8OnOzhRepyPU-t8nZKLzs1pT2JpzP0NS+vVNA@mail.gmail.com
Backpatch: 9.4 only