Dispatcher supports exception handler cancellation and success handling

[IharBury]

Implements dispatcher handling part of #102

• An optional cancellation token is added to the SubscriptionInfo. We have to implement support for it in the event sources yet.
• The memory event source now can also behave as a pure asynchronous event source which does not wait for its subscriptions to process the transactions
• The memory event source now supports the cancellation token in the SubscriptionInfo
• A success handler added to the dispatcher to make it easier to implement exception handling logic based on the history of failures and successes

[IharBury]

Done

[dennisdoomen]

What was the purpose of the Success Handler you added?