STCS cold sstable omission does not handle overwrites without reads

In 2.1, STCS may omit cold SSTables from compaction (CASSANDRA-6109). If data is regularly overwritten or deleted (but not enough to trigger a single-sstable tombstone purging compaction), data size on disk may continuously grow if:

• The table receives very few reads
• The reads only touch the newest SSTables

Basically, if the overwritten data is never read and there aren't many tombstones, STCS has no incentive to compact the sstables. We should take sstable overlap into consideration as well as coldness to address this case.