Fix file creation timing issues (#4)
Fix file creation timing issues (#4)

Refactored and cleaned up data handler

* Data records, which come from the queue are now buffered in a list instead of a growing DataFrame
* a DataFrame is only generated directly before data is written to disk
* writting to file is now delayed for the same time the SNMP query timeout is set so that a new file should be writte approximately at the end of each SNMP query batch 
* no need anymore for the `self.df` DataFrame and the `self.NEW_DataFrame` event
* also added some `self.KEEP_RUNNING == False` for the cases when the
  infinite loops should be stopped

This should close #2 and should close #3. But only the operational long-term application will show if this is really true...
cchwala/pySNMPdaq