At the beginning of a distributed task, move from creating a TChain to
creating an RDatasetSpec to pass to the RDataFrame constructor. This
simplifies the previous logic by avoiding the need for a TEntryList to
restrict the reading of the chain entries to those assigned to the task.

This commit also fixes root-project#10872
Previously, each task received a copy of the names of all
files of the friend dataset. When restricting the processing to a
certain range of entries of the main chain, the friend chain was always
being read starting from the first file. Now, in case there are friend
trees, each task will receive information about the full dataset and
will have to open all files in order to retrieve the number of entries
in all trees. This in turn allows proper alignment w.r.t. the friend
chain.