Change commit_delay to be SUSET for 9.3+

Prior to 9.3 the commit_delay affected only the current user,
whereas now only the group leader waits while holding the
WALWriteLock. Deliberate or accidental settings to a poor
value could seriously degrade performance for all users.
Privileges may be delegated by SECURITY DEFINER functions
for anyone that needs per-user settings in real situations.
Request for change from Peter Geoghegan