Process new emails in batches after login

[bedhub]

As a user with offline storage enabled, i expect that new emails arriving in batches and not one by one after login.

Acceptance Criteria

• newly synchronized mail is not shown one-by-one if there are many
• new emails of the same folder are loaded in one request.

Tasks

• [ ] load multiple mails in a single request when processing updates

Notes part of https://github.com/tutao/tutanota/issues/4170