Fix truncated log output in verbose mode

* when an activity exits quickly with an error, verbose mode would often
  omit the most important last few lines of the log
* reason: flushing the data to disk takes longer than for the Tapasco
  threads to die, thus LogTrackingFileWatcher exits before lines appear
* workaround: when both waitingFor and files are empty, MultiFileWatcher
  now waits one more iteration before exiting, which seems to suffice
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