Fixing issue described in ENG-10144#comment-16a38c06.
Tracked in ENG-10174
fetchOlmAccount creates olm.Account and returns which makes it hard to maintain and free when needed, we should use unpickleAccountAndUseCallback which handles account creation and freeing memory.
Not sure how to handle this case, as I don't feel like updating migration code is something we should do, on the other hand, the entire purpose of my work is to get rid of fetchOlmAccount calls.
Depends on D14320