Improve Akka.Persistence.TestKit

[Arkatufus]

Improve Akka.Persistence.TestKit

Changes

• Allow for external pre-instantiated ActorSystem to be passed in into the constructor
• Make all of the interceptors API public instead of internal to allow custom interceptor composition
• Add a new ConnectionInterceptor that intercepts mocked database connection, emulating connection failures
• Add a new interceptor CancelableDelay that is less prone to strict timing failure, a delay stops immediately when cancelled, lessening dependency on fragile timing issues.

[Aaronontheweb]

LGTM