Consider coldness in STCS compaction

I see two options:

1. Don't compact cold sstables at all
2. Compact cold sstables only if there is nothing more important to compact

The latter is better if you have cold data that may become hot again... but it's confusing if you have a workload such that you can't keep up with all compaction, but you can keep up with hot sstable. (Compaction backlog stat becomes useless since we fall increasingly behind.)