Cherry-pick bot is discarding valid event listeners

In some cases of changes in a relation chain, the cherry-pick bot fails to stage the change when the parent change stages automatically.

This appears to be due to rejection of listener setup due to duplicate event names. This rejection is intended to avoid duplicating work, but in practice means that events which rely on the same event are mutually exclusive.

The RelationChainManager has logic to post a comment to a change if the parent is not staged after 48 hours in order to prompt manual attention, but this listener event also sets up a cancellation trigger to avoid posting the message if staging does occur. As a result, the event staged_[changeId] is reserved for the cancel event, and any further subscriptions to this event are ignored. Since the same event is relied upon to automatically trigger staging of the child change, this does not happen, but the warning message is cancelled as expected.

Fix toolbox.js::setupListener to check all arguments passed and disallow complete duplicates.