Re: Allow parallel plan for referential integrity checks?

I looked at your patch and it's a good idea to make foreign key validation
use parallel query on large relations.

It would be valuable to add logging to ensure that the ActiveSnapshot and TransactionSnapshot
is the same for the leader and the workers. This logging could be tested in the TAP test.

Also, inside RI_Initial_Check you may want to set max_parallel_workers to
max_parallel_maintenance_workers.

Currently the work_mem is set to maintenance_work_mem. This will also require
a doc change to call out.

/*
* Temporarily increase work_mem so that the check query can be executed
* more efficiently. It seems okay to do this because the query is simple
* enough to not use a multiple of work_mem, and one typically would not
* have many large foreign-key validations happening concurrently. So
* this seems to meet the criteria for being considered a "maintenance"
* operation, and accordingly we use maintenance_work_mem. However, we