- bugfix: solved potential (temporary) stall of messages when the queue was
  almost empty and few new data added (caused testbench to sometimes hang!)
- fixed some race condition in testbench
- added more elaborate diagnostics to parts of the testbench
- solved a potential race inside the queue engine

made shutdown more reliable by makeing sure that the main queue DA worker
is only cancelled if this is actually unavoidable. Also moved down the
deletion of rsyslogd's pid file to immediately before termination, so
that absence of the file is a proper indication that rsyslogd has
finished (in the past, e.g. the testbench accidently ran two intances
as the pid file was deleted too early). Also some improvments to the
testbench, namely to handle aborts more intelligently (but still not
perfect).

Some devices seem to create them and I do not see any harm in supporting that.