BUG: Multiple subscriptions to the same method in a cluster only triggers the first callback.

In the subscribe() function of the electrum cluster, we set up a subscriptionResponder which is used to validate integrity of notifications before calling the provided callback function.

This function uses the local contextually available callback, and as such is the same every time.

We add this function to our event listener, but only if it has not already been added. As a result, it only gets added once, and it resolveds to the context of the first execution.

Ideally we it should be uniquely identifiable and be added to the listeners every time.