Added option to customize partition events by stage logic subclasses

[IgorFedchenko]

This is a small PR, which adds a way to customize partition events handling by different stage logic classes.

Before this PR partitions events were handled by SingleSourceStageLogic without any option to override it. Besides, original kafka driver callbacks also provide consumer instance passed to callbacks, so actually handling could use some consumer's methods.

Now consumer is passed to this callbacks, and the callbacks itself may be customized by logic subclasses (looks like only for TransactionalSourceLogic so far).

[IgorFedchenko]

During PR #48 review process, I thought that IPartitionEventHandler interface will be made public. But currently all references in original alpakka project, that use this interface, are private/internal. So actually making it public will not give library users any option of use this interface - so keeping it internal so far. We can switch it to public whenever there will be any option to use it from outside.

[Aaronontheweb]

Looks good to me, but left 1 question

[Aaronontheweb]

LGTM

[Aaronontheweb]

@IgorFedchenko FYI, on the previous commit had one test failure:

Akka.Streams.Kafka.Tests.Akka.Streams.Kafka.Tests.Integration.PlainSourceIntegrationTests.PlainSource_consumes_messages_from_KafkaProducer_with_subscribe_to_topic

You can see the logs and such if you click through here: https://github.com/akkadotnet/Akka.Streams.Kafka/runs/227053848

Might have a race condition of some kind with that test.

[IgorFedchenko]

@Aaronontheweb Good point. Created issue for that - looks little bit wierd, so unfortunately can not fix this right now, "on the fly".