Add and use dht::compaction_group_of that computes the compaction_group index by unbiasing the token, similar to dht::shard_of. This way, all tokens in `_compaction_groups[i]` are ordered before `_compaction_groups[j]` iff i < j. Fixes #12595 Signed-off-by: Benny Halevy <bhalevy@scylladb.com> Closes #12599