Split off from issue598

Merged.

Fine with me.

With Jendrik's second review, can we merge this without experiments?

I had a look as well and everything looks good.

I looked at the code. this was well worth moving to a separate issue. I have no
further comments, but I cannot guarantee that I missed nothing, so if you want a
bit more certainty, we should either have experiments or someone else reviewing
the code.

I created a pull request for all of the changes from issue598 that we don't want to 
handle there:

https://bitbucket.org/FlorianPommerening/downward-issues/pull-
requests/24/issue676/diff

As a preparation for issue598, we want to bring the PDB up to date with our new 
guidelines on how and when to pass a task. This will also touch on some global 
classes that are used by the PDB code (SuccessorGenerator and VariableOrderFinder), 
but except for those, I'd like to limit the diff to the "pdbs" directory. We should 
think about creating similar issues for other parts of the code.