simplified and thus speeded up the queue engine, also fixed some
potential race conditions (in very unusual shutdown conditions)
along the way. The threading model has seriously changes, so there may
be some regressions.
NOTE: the code passed basic tests, but there is still more work
and testing to be done. This commit should be treated with care.
- 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
... and also improved the test suite. There is a design issue in the
v3 queue engine that manifested to some serious problems with the new
processing mode. However, in v3 shutdown may take eternally if a queue
runs in DA mode, is configured to preserve data AND the action fails and
retries immediately. There is no cure available for v3, it would
require doing much of the work we have done on the new engine. The window
of exposure, as one might guess from the description, is very small. That
is probably the reason why we have not seen it in practice.
