Fix denseness and column metadata updates coming from Thrift

It was decided that we'd be recalculating is_dense for table updates coming from Thrift on every change. However, due to some oversight, is_dense can only go from false to true. Once dense, even adding a REGULAR column will not reset is_dense back to false.

The recalculation fails because no matter what happens, we never remove the auto-generated CLUSTERING and COMPACT_VALUE columns of a dense table.

Which ultimately leads to the issue on 2.2 to 3.0 upgrade (see CASSANDRA-11315).

What we should do is remove the special-case for Thrift in LegacySchemaTables::makeUpdateTableMutation and correct the logic in ThriftConversion::internalFromThrift to remove those columns when going from dense to sparse.

This is not enough to fix CASSANDRA-11315, however, as we need to handle pre-patch upgrades, and upgrades from 2.1. Fixing it in 2.2 means a) getting proper schema from DESCRIBE now and b) using the more efficient SparseCellNameType when you add columns.