Uninterruptible iterations

[na--]

When ramping-down VUs in a stages config, with the current k6 implementation, some iterations may be interrupted midway through their execution. In some situations that may not be ideal, since it'll make comparison of metrics between iterations pretty difficult. So, ideally, we should have an option that causes k6 to wait for iterations to complete when ramping down VUs.

[na--]

For posterity, this has been implemented as part of #1007. It is a property of the new executors there called gracefulStop (for waiting iterations to complete at the end of the test) and gracefulRampDown (for ramping down then there are stages). By default both of these values would be 30s.