Optimize truncate when snapshots are disabled or keyspace not durable

My goal is to make truncate to be less IO intensive so that my junit tests run faster (as already explained in CASSANDRA-3710). I think I have now a solution which does not change too much:

I created a patch that optimizes three things within truncate:

• Skip the whole Commitlog.forceNewSegment/discardCompletedSegments, if durable_writes are disabled for the keyspace.
• With CASSANDRA-3710 implemented, truncate does not need to flush memtables to disk when snapshots are disabled.
• Reduce the sleep interval

The patch works nicely for me. Applying it and disabling durable_writes/autoSnapshot increased the speed of my testsuite vastly. I hope I did not overlook something.

Let me know if my patch needs cleanup. I'd be glad to change it, if it means the patch will get accepted.