Adapt to changes in core from briar!1775

Solve the more general problem where a task already running on the event
executor schedules another task on the same executor and expects that to
finish asynchronously. With the single-threaded event executor this does
not work unless such tasks as immediately executed instead of being put
into the queue.

see https://github.com/JetBrains/compose-jb/issues/2729

fixes #355

fixes #485