[8.x] Run observer callbacks after database transactions have committed

[bakerkretzmar]

This PR sets public $afterCommit = true on Scout's ModelObserver class, so that the saved, deleted, forceDeleted, and restored model event callbacks, if they are triggered inside a database transaction, are not executed until that transaction has committed.

Background

This fixes an existing issue where incorrect data is synced to a search index if a model event Scout listens for, such as saved, is fired inside a database transaction.

If Scout is not set up to queue search index updates, this issue affects all code that fires the above model events inside database transactions, because Scout's observer method is executed immediately—before the transaction has committed or been rolled back—syncing stale or invalid data.

If Scout is set up to use the queue this issue will appear intermittently, when the queue workers pick up and execute the syncing job before the transaction has committed or rolled back. In my experience this happens almost all the time.

See #152 and laravel/nova-issues#1906.

It wasn't really possible to address this at all up until now, but transaction-aware code execution was just added to Laravel core (see laravel/framework#35373 and laravel/framework#35434). In Laravel 8.17.0 and later, it's trivial to delay any listeners and observers until open database transactions have committed.

Backwards compatibility

Theoretically this could change some behaviour in existing Scout installations, but I would argue that that behaviour is a bug and should be fixed. I can't imagine a situation where an application would be relying on stale or invalid data being persisted to a search index, so I doubt this change would break anything.

Tests

Since model observers are basically just larger listener classes, in my opinion this PR is covered by Laravel's tests for delaying listener execution until transactions have committed: https://github.com/laravel/framework/blob/8.x/tests/Integration/Events/ListenerTest.php.

[driesvints]

I've sent in a PR to revert this because this causes test suite that have the DatabaseTransactions trait applied to fail. We can make this change in the next major release. Feel free to send in a PR to master.