LegacyLayout errors on collection tombstones from dropped columns

When reading legacy sstables in 3.0, the dropped column records in table metadata are not checked when a collection tombstone is encountered. This means that if a collection column was dropped and a new column with the same name but a non-collection type subsequently added prior to upgrading to 3.0, reading any sstables containing the collection data will error. This includes reads done by upgradesstables, which makes recovery from this situation without losing data impossible. Scrub will clean the affected tables, but any valid data will also be discarded.