Breaking change Wishlist for 4.0

This ticket is for general discussion of the breaking changes in the coming 4.0 release.

Please edit the main post to add a topic with a sequence number, and we can discuss it in the comments. If the breaking change is decided to be worthy to add, then create a separated issue for it.

1. [POSTPONED] Do not allow having both typed Realm instance and Dynamic Realm instance on the same thread. Throw an exception instead. This creates lots of troubles with Realm instance cache management. By disallowing it, the RealmCache implementation can be much simpler and give us an option to totally manage the Realm instance through Object Store.

2. [POSTPONED] Change the behavior of distinct() to work against query result. See #4774

3. [DONE] Replace RxJava1 with RxJava2. This issue here #3497 describes an approach where we split RxJava support into a separate library, but after trying that out in a sample project, it just feels really clunky. Doing realm.where(Person.class).findAll().asFlowable() is just so much nicer than doing RxRealm.asFlowable(realm.where(Person.class).findAll()) and if one thing Kotlin has proved, then it is that small things matter 😄

4. [POSTPONED] Remove the mips so file since there is NO mips device in the market.

5. DONE [NH] Rename getAccessToken either rename it to reflect the fact that it’s returning a refresh_token, or abstract the name to the end user.

6. DONE [NH] Remove deprecated methods retrieveUser and retrieveUserAsync as well as the deprecated SyncUser#Callback

[beeender]

Do not allow having both type Realm instance and Dynamic Realm instance on the same thread.

Yes, for the same RealmConfiguration! Migration is different where we control the Realm instance internally.

[cmelchior]

Feature set for 4.0 is complete. Moving the rest of the items to #5372. Closing