Fix segfault when scheduling EventViews

Scheduling sub-slots/EventViews was moved to the scheduler actions queue in !729 (merged). While this protects against thread-unsafety in the bookkeeping, it introduced the possibility of the source EventContext being deleted before the action depending on it was popped from the queue.

Since the only information required from the source context was the slot index, this information is captured by value in the lambda instead.

See #48 (closed) and discussion on https://gitlab.cern.ch/atlas/athena/merge_requests/14713

Regards, Ben