Update the tablet metadata save and read methods to work with tablet
maps of co-located tables.
The new function colocated_tablet_map_to_mutation is used to generate a
mutation of a co-located table to system.tablets. It creates a static
row with the base_table column set with the base table id. The function
save_tablet_metadata is updated to use this function for co-located
tables.
When reading tablet metadata from the table, we handle the new case of
reading a co-located table. We store the co-located tables relationships
in the tablet_metadata_builder's `colocated_tables` map, and process it
in on_end_of_stream. The reason we defer the processing is that we want
to set all normal tablet maps first, to ensure the base tablet map is
found when we process a co-located table.
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