A SubscriptionGuarantor maintains a set of pending subscriptions,
resending the subscribe command unless and until the subscription
is confirmed or rejected by the server or cancelled client-side.
A race condition in the ActionCable server - where an unsubscribe
is sent, followed rapidly by a subscribe, but handled in the reverse
order - necessitates this enhancement. Indeed, the subscriptions created
and torn down by Turbo Streams amplifies the existence of this race
condition.
