We currently have code to calculate "is_dense" in the create statement handler.
That obviously don't work for the system schemas, which are not defined this
way.
Since all of our schemas now have to pass through the schema_builder one way or
another, that is the best place in which to do that calculation.
Note that unfortunately, that does not mean we can just get rid of
set_is_dense() in the schema builder: we still need to set it in some
situations, where for instance, we read that property in schema_columnfamilies,
and then apply to the relevant CF. Those uses are, however, all internal to
legacy_schema_tables.cc
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>