LDEV-5083 Trigger Flux update on each activity entrance
Till now the update was triggered on activity completion. It is a better
approach, but it only worked on finishing tool activities. Gates and,
for example, Leader Selection activity were skipped and would not
trigger learner progress update.
Now update gets triggered on learner joining a lesson and traversing
through each activity, which is good enough. Emitted items get throttled
anyway.
