Understand how to properly use tbb:task_arena

The following discussion from !2381 (merged) should be addressed:

• @nnolte started a discussion: (+5 comments)

  i am hacking a bit here..
  with the global tbb threadpool, we were able to schedule a mutable lambda.
  however, the execute happening within a task_arena works with a const F,
  so that didn't work. As a workaround, I have this explicit callable where only the eventcontext is mutable.
  Not sure what the reason for const F is, so circumventing this with mutable EventContext does not seem like my best idea yet..