CQL3: move {max/min}_compaction_thresholds to compaction options

It makes way more sense to have min_compaction_threshold and max_compaction_threshold be parts of the compaction_strategy_options. They are not in thrift (and CQL2) only for historical reasons, but there is no reason not to fix it. Especially given that they don't make sense for all compaction strategy (Leveled compaction ignores them).