[DF] Input TFile's are closed at destruction instead of the end of the event loop

In general, RDataFrame should not hog file descriptors by keeping files open after the event loop. In particular, this is a problem when running some of our gtests on windows, as the tests cannot delete the files produced because RDataFrame is still "using" them (and on windows it's not possible to delete a file if someone else has a handle to it).