Re-encryption should handle task cancellation and progress better

Re-encryption should handle task cancellation and progress tracking better in general.

In a recent internal report, a canceled re-encryption could lead to the progress of the zone being 'Processing' forever. Sending a new cancel command would make it complete, but new re-encryptions for the same zone wouldn't work because the canceled future is not removed.

This jira proposes to fix that, and enhance the currently handling so new command would start from a clean state.