improve worker thread pool handling

among others, make possible that workers really timeout and the pool
thus shrinks
doc: mention Pavel's improvement in ChangeLog
Merge branch 'master' into master-newwtipool
cleanup no longer needed code
bugfix: regular workers are no longer started if DA worker is active

form mail:
Look, below, if it is time to activate DA worker, we call it explicitly. But in this case we do not advise regular workers. They are likely already running at this point, but it is not guaranteed. What if, for example, the system is set to start additional workers when the queue is going over high watermark? What if HighWatermark is set to 1? Regular workers will not be started, and DA worker may fail. Thus, it is reasonable to advise regular workers even if we are going DA.
doc: maintain ChangeLog