[FEAT ember-data] unloadRecord and unloadAll should return promises

[runspired]

these promises would resolve once unloading is complete. The timing of what occurs in an unload would be unchanged for now. Today, these methods have no return value.

[Alonski]

Would this change need to happen in all of these places:
https://github.com/emberjs/data/blob/4bcf1f98e51708bb6c9eab82e8c29bfd5c6813da/addon/-private/system/model/model.js#L626
https://github.com/emberjs/data/blob/4bcf1f98e51708bb6c9eab82e8c29bfd5c6813da/addon/-private/system/model/internal-model.js#L423
https://github.com/emberjs/data/blob/4bcf1f98e51708bb6c9eab82e8c29bfd5c6813da/addon/-private/system/model/record-data.js#L329

Trying to figure out which one is the one called by users when they do store.unloadRecord

[runspired]

not interested in this anymore due to unload APIs being re-evaluated in favor of store forking and automated GC. If we do retain any unload APIs its likely we do a broader GC related RFC. The "delete" use cases is now handled by operations in #854