Add a new column base_table to the system.tablets table.
It can be set to point to another table to indicate that the tablets of
this table are co-located with the tablets of the base table.
When it's set, we don't store other tablet information in system.tablets
and in the in-memory tablet map object for this table, and we need to
refer instead to the base table tablet information. The method
get_tablet_map always returns the base tablet map.
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