Cancelling a Queen late in a build causes negative index exception

[bsaffel]

This appears to be because when the queen was cancelled, the unit was found to be at benchmark index 6, which based on the current offset would return a negative objective index since that had long since been cycled out.

Need to update the logic around how units cancelled later do not attempt to cancel when they are no longer valid to be tracked or have no associated step.

[bsaffel]

Added a guard so that steps that are no longer objectives are ignored when cancelled.

Still need a way to prevent previous, visible, and valid steps from being cancelled when a new unit over a threshold is cancelled though.

[bsaffel]

Probably depends on #314 so that evaluation can be done based on number of steps for a given unit rather than supply. It makes the logic drastically simpler and more flexible.

[bsaffel]

Not seen recently despite multiple testing scenarios