Add the ability to determine if a Synchronization instance is part of a given Exchange instance

The current Exchange implementation does not provide the ability to determine if a given Synchronization instance is already part of a given Exchange.
There is a request that we provide the ability to determine if a given Synchronization instance is part of an Exchange instance.

Currently, the only method that we have is the 'addOnCompletion' call.

We could include a 'allOnCompletion' call that returns a unmodifiable collection or we can have a 'containsOnCompletion' method that returns true if the instance exists.