Fix counting of tombstones towards TombstoneOverwhelmingException

CASSANDRA-6042 introduced warning on too many tombstones scanned, then CASSANDRA-6117 introduced a hard TombstoneOverwhelmingException condition.

However, at least SliceQuerFilter.collectReducedColumn() seems to have the logic wrong. Cells that are covered by a range tombstone or a partition high level deletion, still count towards ColumnCounter's ignored register.

Thus it's possible to have an otherwise healthy (though large) dropped partition read cause an exception that shouldn't be there.

The only things that should count towards the exception are cell tombstones and range tombstones (CASSANDRA-8527), but never ever live cells shadowed by any kind of tombstone.