Unsubscribing an event store subscription should not delete the subscription. This pull request adds Commanded.EventStore.delete_subscription/2 to allow an existing subscription to be completely removed.

Example usage

Create a subscription to all events:

{:ok, subscription} = Commanded.EventStore.subscribe_to(:all, "Example", self())

Unsubscribe the subscription to stop receiving events, but still remember the subscription's last acknowledged event pointer:

:ok = Commanded.EventStore.unsubscribe(subscription)

Finally, delete the subscription:

:ok = Commanded.EventStore.delete_subscription(:all, "Example")