As of 2.8.0-rc3, WT is not configured to use synchronous logging. That is configured using the WT transaction_sync flag when the engine is opened, or can be controlled on a per-operation basis using the j:true write concern.
Currently, the journal is only fsynced when a journal file fills up and rolls over to the next one, or a j:true write is made (it sweeps up all writes before it).
There needs to be a knob to control when the journal is fsynced. Makes sense to do this using the existing journalCommitInterval which would trigger a dummy transaction