Ensure Subscriptions are successfully added to Topics on reconnect.

Addresses issue #1144

Since the SCTS.Cancel uses UnsafeQueueUserWorkItem, old Subscriptions
are often not removed before new ones are added when reconnecting.
Topic.AddSubscription would then block the addition of reconnecting
Subscriptions, since reconnecting Subscriptions have the same Identities as
the yet undeleted old subscriptions.

This change removes the check inside of Topic.AddSubscription preventing
two Subscriptions with the same Identity from being subscribed to the topic
at once.