Support for write with unless_exist: true

[mintuhouse]

I am implementing a simple lock interface on Rails.cache
& currently using Rails.cache.fetch(lock_key, expires_in: timeout) { lock_value } == lock_value
But it is not atomic as it makes multiple SQL queries

Can we implement support for Rails.cache.write(lock_key, lock_value, unless_exist: true, expires_in: timeout)

[djmb]

Should be fixed by #188, but that will have to wait for support for Rails 7.1 and earlier to be dropped.

[skatkov]

@djmb can we have two side-by-side?

In case Rails 7.1 unless_exist version will be used, if version is older it will use different class.

[djmb]

@skatkov - I've had to rework the PR to handle expired entries, which had the side effect of removing the dependency on Rails 7.2+ so we'll be able to merge that now.