Avoid hot standby cancels from VAC FREEZE

Avoid hot standby cancels from VAC FREEZE

VACUUM FREEZE generated false cancelations of standby queries on an
otherwise idle master. Caused by an off-by-one error on cutoff_xid
which goes back to original commit.

Analysis and report by Marco Nenciarini

Bug fix by Simon Riggs

This is a correct backpatch of commit 66fbcb0d2e to branches 9.1 through
9.4.  That commit was backpatched to 9.0 originally, but it was
immediately reverted in 9.0-9.4 because it didn't compile.