collect_uprobes.py

- When disabling uprobes, check if the directory exists before
  trying to iterate.
- So far, collecting the uprobes simply worked as an infinite
  loop. If we want to measure a certain scenario, however, we
  want to be able to stop the collection after the use case
  has finished executing. Allow this by passing in an
  additional parameter which defines a maximum number of rounds
  that should be run without a change. If that number is reached,
  we exit the collection process.