merge EventLoop Worker implementations

So far there has been a separate Worker class for each driver, leaving us with three worker implementations: DirectWorker, BatchWorker and GridWorker. Since by now most of the functionality was in the base class and the individual implementations added only little on top it seemed reasonable to just merge them all together into one class.

Right now this is a bit messy, but at least all the code is now in one place and I can hopefully start to consolidate and restructure it some more, cleaning up the mess in the process. I also should merge the different worker run scripts/executables into a single script/executable.

Besides just cleaning up and consolidating the code base, one of the goals is to develop a single clean interface for running an EventLoop worker inside a docker container, which can then be used by the batch drivers, the grid driver and third party tools. But that is still a bit off, and will probably come in a separate merge request.