recover from RecordNotUnique race condition when creating new legacy appeal

[tomas-nava]

Description

If LegacyAppeal.find_or_create_by_vacols_id is hit multiple times for an appeal that doesn't yet exist, it's possible for a race condition to arise where the appeal is saved by one process in between the time another process initializes and attempts to save it.

I observed this happening when loading the case details page for appeals that exist in VACOLS but not Caseflow, when find_or_create_by_vacols_id is invoked from the appeals controller and the tasks controller in quick succession. When the second process attempts to save its version of the appeal, it fails with PG::UniqueViolation / ActiveRecord::RecordNotUnique.

I changed the method to capture the ActiveRecord::RecordNotUnique and return the existing record via find_by! (which'll raise a ActiveRecord::RecordNotFound if it's unsuccessful), following this pattern, from the Rails 6 create_or_find_by method.

It's not ideal to use an exception as flow control, but I think it's okay in this case to let the database constraint determine the action.

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[ferristseng]

🎉 LGTM!