Deleting a replay does not stop its task

When [deleting a replay](https://docs.influxdata.com/kapacitor/v1.4/working/api/#delete-replay), the replay is effectively removed from the list of replays, but its task keeps running until the data is exhausted.

Consequently, there is currently no way to cancel a replay and replace it with a new one - depending on the task pipeline, we either get two pipelines running at the same time or the second pipeline encounters an error (for example, if pipeline contains an httpOut node, it will complain about multiple registrations).

It seems there is already a [TODO in the code](https://github.com/influxdata/kapacitor/blob/f0ca096361214004d65cc12e30aa4c096501421e/services/replay/service.go#L809) - is anyone working on this at the moment?