WAIT FOR command should do some query jumbling

[Peter Eisentraut]

Right now, WAIT FOR commands are recorded literally in 
pg_stat_statements.  I think the actual LSN value should be replaced by 
some placeholder, like

WAIT FOR LSN $1
Otherwise, a client library or framework that runs this command 
repeatedly would needlessly spam pg_stat_statements.

The query jumbling facilities should be used to handle this.