Allow the application to call ensureReadyForSyncing() and doOneSyncCycle() to gain finer control to the sync process. For example, they can decide when to retry if there is a failure, instead of relying on the fixed logic in syncForever(). There is no behaviour change for applications.

Currently, these functions offer no particular interest to applications because the login functions will still call startSyncing(). In the future, we will add other login functions that do not call startSyncing() to replace the current functions. However, this change is still useful for unit tests, because it allows us to assert model changes after "one sync cycle."

CCBUG: https://iron.lily-is.land/T110

Type: skip

Test kazv works the same way as before.