Use a set of worker threads to push sstables.
Use a set of worker threads to push sstables.

Remove the explicit backup option as there's no easy way to handle the
race condition during startup while Cassandra is running. Always add
all sstables to the flush list.

russss/tablesnap,tamsky/tablesnap,apigee/tablesnap,ethanmiller/tablesnap,curalate/tablesnap,JeremyGrosser/tablesnap,juiceblender/tablesnap,betable/tablesnap,heroku/tablesnap,betable/tablesnap,russss/tablesnap,apigee/tablesnap,JeremyGrosser/tablesnap,juiceblender/tablesnap,amorton/cassback