[HACKERS] patch: optimize information_schema.constraint_column_usage

Hello hackers,

The view information_schema.constraint_column_usage becomes slow when 
the number of columns and constraints raise to substantial values.
This is because of a join condition that allows only join filter to 
enforce. The patch is to optimize it.
See many_constraints.sql file attached for a performance test: create 
3000 tables with 10 columns and a PK each and select * from the view.
The last statement works for 22 seconds on master branch, 34 
milliseconds optimized on my laptop.

Best Regards,
   Alexey Bashtanov

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