Mattermost-server: Migrate "Team.GetChannelUnreadsForAllTeams" to Sync by default

[esethna]

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Notes: Jira ticket

Mattermost is migrating its Store layer to be sync by default, and only use Async when needed and we're looking for contributors to help with that effort. This Help Wanted issue is to migrate the GetChannelUnreadsForAllTeams in the Team store.

The expected way to implement it is, go to the Team store implementation in the store/sqlstore/ directory, modify the method GetChannelUnreadsForAllTeams to return directly an object from the model module, and a *model.AppError (removing the store.Do wrapper). After that, you must modify the interface defined in store/store.go to match with the changes. Then, you should execute make store-mocks to rebuild the mocks with the new interface. And finally, modify the rest of the code (tests included) to use the new interface of the function properly.

Example: mattermost/mattermost-server#10613

[jesperhansen17]

Can I take this one?