system_update_8402 does not affect media entities

Re-titled, and tagged.

And, the patch.

Here's how to test this:

1. Start with Media Entity 1.x on Drupal 8.3.x.
2. Run the update path, like normal.
3. Watch the order of executed updates carefully. If system_update_8402 runs after media_entity_update_8201, you will be able to reproduce this bug. Without hook_update_dependencies(), the updates may run in a non-deterministic order, so it is possible you won't see this bug. However...
4. Check all base tables for the media entity type -- you should have a revision_affected_translation column in one of them. If this bug has reared its ugly head, you won't. But with this patch applied, you will.

Bumping status to critical, since this blocks the update path.

This bug is tricky to reproduce, because it is non-deterministic in nature. It depends on updates running in the wrong order, which is basically impossible to guarantee.

I just explained this to @seanB over the phone, but I want to record it here for future reference.

Here's the thing: in order to prevent things from breaking during the update process, the system must, at all times, be aware of the media and media_bundle entity types -- at least until core Media is installed, since it canonically defines those entity types. This is why the 2.x version of Media Entity implements hook_entity_type_build(). It's a shim to make sure the system always knows that media and media bundle entities exist, even when core Media is not installed.

Now: along comes system_update_8402, which collects all entity type definitions in order to determine which ones need the revision_translation_affected field added. hook_entity_type_build() is invoked as part of this process. However, because the hook has just been added to Media Entity's code base, the module handler is not aware that Media Entity now implements hook_entity_type_build(), because it is relying on a stale hook implementation cache! So media_entity_entity_type_build() is never called, and the system never knows that the media entity type exists. Therefore, it does not add the revision_translation_affected field to media entities, and things blow up later. You can get around this problem by clearing all caches before running database updates, but we most definitely cannot guarantee that users will remember to do that.

So the only real fix here is to add an additional update hook which runs before system_update_8204 and guarantees that the hook implementation cache is primed with the latest information.

That's what this patch does. It adds media_entity_update_8200, which clears the implementation cache, and then defines a dependency that forces it to run before system_update_8204.

What makes this so hard to verify is the fact that, absent a dependency graph, updates run in a non-deterministic order. So it is entirely possible that, even without the dependency, media_entity_update_8200 would run before system_update_8204, thus accidentally avoiding this problem. That's not good enough: we need to be damn sure that media_entity_update_8200 runs before system_update_8204.

After discussion with @seanB on IRC, we decided it would be wise to add a hard dependency that forces media_entity_update_8201 to run after system_update_8402, for the reasons mentioned in #7.

Committed and pushed. Thanks!