Allow consumers to control start from beginning behavior

[alisaifee]

Clients of the multi-process consumer currently have no way of enforcing that the consumer resumes from the last committed offset. SimpleConsumer automatically receives this behaviour when auto_commit is enabled and can explicitly request the offset using fetch_last_known_offsets.

To ensure backward compatibility a start_from_beginning parameter has been added to the base consumer which defaults to True.

Reference: #347

[alisaifee]

The failing test is unrelated. Theres a new PR #354 which pretty much does the same thing (with inverted logic for the default value of the kwarg for SimpleConsumer). I'm ok with closing this if #354 is preferred.

[vshlapakov]

Oops, my apologies @alisaifee, I missed your PR (searched by word MultiProcessConsumer and found earlier hanging PR #252). They both do practically the same and I'll be fine too if your PR is preferred, it's be just great to add this functionality asap.

[dpkp]

I think there's a better approach to this -- see #356 . I cherry-picked your test commits :)

[alisaifee]

👍
closing this PR.
Thanks!