resetTango should validate and clean up the machines dictionary

[cg2v]

I have encountered a couple of situations in testing where the preallocator's machines datastructure got out of sync. There were machine ids in the list that no longer had an entry in the relevant queue, which prevented any jobs from being scheduled on that machine. If all machines get in that state, job scheduling would halt. #77 fixes some of these, but it probably makes sense to either reset the preallocator state or validate it when tango is reset.