The method all_tables in tablet_metadata is used for iterating over all
tables in the tablet metadata with their tablet maps.
Now that we have co-located tables we need to make the distinction on
which tables we want to iterate over. In some cases we want to iterate
over each group of co-located tables, treating them as one unit, and in
other cases we want to iterate over all tables, doesn't matter if they
are part of a co-located group and have a base table.
We replace all_tables with new methods that can be used for each of the
cases.
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