no longer cause launching of instances multiple times for the same job; use a redis lock to quickly find out if an instance is still starting up before trying to terminate it
new ec2_spot_price_percent and ec2_spot_force_percent options and the ec2 scheduler implements spot requests; launch_instances() no longer returns failed instances, and no longer waits forever on broken instances that are non-responsive to ssh; instances are not terminated based on a redis key that says we recently tried to launch it