Provide option to avoid loading orphan SSTables on startup

When Cassandra starts up, it enumerates all SSTables on disk for a known column family and proceeds to loading all of them, even those that were left behind before the restart because of a problem of some sort. This can lead to "data gain" (resurrected data) which is just as bad as data loss.

The ask is to provide a yaml config option which would allow one to turn that behavior off by default so a cassandra cluster would be immune to data gain when nodes get restarted (at least with Leveled where Cassandra keeps track of SSTables).

This is sort of a follow-up to CASSANDRA-6503 (fixed in 1.2.14). We're just extremely nervous that orphan SSTables could appear because of some other potential problem somewhere else and cause zombie data on a random reboot.