Wipe out pre-existing problems before running transaction (#678201, pmatilai).
    
    - It wipes out any pre-existing problems (basically dependency problems
      found in ts.check(), which cannot be filtered with ts.setProbFilter()
      (stupid as it is). Which should solve #678201.
    - It also lets rpm free up dependency-related data which is not needed
      during the actual transaction so anaconda will use a bit less memory.