revisit if thread marked as unkillable is okay to be killable for serverless related

In SERVER-70127, we changed the default behavior of a thread to be killable by default. We now marked all threads that were not marked as killable before to be unkillable, but it could be there are threads not necessary to be unkillable.

This ticket is created for teams owning relative code directories to revisit whether these threads should be marked as unkillable.

You can search the ticket number(SERVER-74661) to lookup TODO threads.