Change delimiter used for display of NextXID

Change delimiter used for display of NextXID

NextXID has been rendered in the form of a pg_lsn even though it
really is not. This can cause confusion, so change the format from
%u/%u to %u:%u, per discussion on hackers.

Complaint by me, patch by me and Bruce, reviewed by Michael Paquier
and Alvaro. Applied to HEAD only.

Author: Joe Conway, Bruce Momjian
Reviewed-by: Michael Paquier, Alvaro Herrera
Backpatch-through: master