Compaction degrades key cache stats

When "compaction_preheat_key_cache" is set to true, then during compaction, it keep tracks of cached keys to to re-cache their new position.
It does this by calling the following method on every key of the compacted sstable :
sstable.getCachedPosition(row.key)
which also update cache stats, thus lowering hit rate

Below is an attached patch allowing to know if the key is cached, but without updating the stats.