SourceWithOffsetContext

[IgorFedchenko]

As a part of work over issue #36 , SourceWithOffsetContext consumer source will be implemented and tested in this PR.

This souse is basically provides another way of passing consumed messages and committable offset to downstream. The main difference (as far as I understand) is that while CommittableSource passes CommittableMessage to the downstream, forcing consequent flows to handle both message and it's committable offset, this SourceWithOffsetContext allows to put committable offset to the source's context.

Also, some helpers like Committer static class with committing flows are implemented in this PR, because they are used to make testing simpler/cleaner.

There is one more test for this source type, but it will require one more producer flow implementation, so it will be added in one of the next PRs.

[Aaronontheweb]

@IgorFedchenko have some merge conflicts here

[IgorFedchenko]

Yes, each of my PRs adds new extension method to KafkaConsumer.cs file, so seems like we will have conflicts on each merge...

[Aaronontheweb]

@IgorFedchenko ah, ok

[Aaronontheweb]

Looks good overall, but have some questions

[IgorFedchenko]

After updating committer default settings in reference.conf file one of the tests failed because I was expecting 10 messages on the downstream, but there are less of them due to batchSize > 1 (so now expecting elemsCount/batchSize committable batches).