Move fetching last known offset logic to a stand alone function.

The `Consumer` class fetches the last known offsets in `__init__` if
`auto_commit` is enabled, but it would be nice to expose this behavior
for consumers that aren't using auto_commit. This doesn't change
existing behavior, just exposes the ability to easily fetch and set the
last known offsets. Once #162 or something similar lands this may no
longer be necessary, but it looks like that might take a while to make
it through.

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