Fix database race condition when tracking OTP requests

[mitchellhenke]

OtpRequests has a unique index on [phone_fingerprint, phone_confirmed], so we get exceptions (NewRelic link) due to the race condition when two requests try to create the row in a short time period.

This PR makes the change use create_or_find_by which attempts to create the row and rescues if the unique index if the unique constraint is violated and then issues a select to get that row. This is similar to the work done in #5341

There is a slight behavior change in this in that the row will have nil for otp_last_sent_at when it is initially created, but this seems to be safe at first glance since we call increment when an OTP is sent.

[zachmargolis]

LGTM, this looks like it reduces test coverage but we're relying on this new create_or_find to itself rely on DB indexes and be smarter, right?

[zachmargolis]

we can just inline this now, right?

[mitchellhenke]

yep

[mitchellhenke]

LGTM, this looks like it reduces test coverage but we're relying on this new create_or_find to itself rely on DB indexes and be smarter, right?

create_or_find_by is new in Rails 6, and has similar behavior with hopefully less lines. It does rely on the unique database index to rescue the create if it fails on the unique constraint so it can fall back to the find_by.