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

Yelp/kafka-python,grue/kafka-python,ecanzonieri/kafka-python,zackdever/kafka-python,ianawilson/kafka-python,docker-hub/kafka-python,gamechanger/kafka-python,trbs/kafka-python,ohmu/kafka-python,xiaosl/kafka-python,nmandavia/kafka-python,CrowdStrike/kafka-python,saaros/kafka-python,mumrah/kafka-python,docker-hub/kafka-python,ohmu/kafka-python,jessamynsmith/kafka-python,dyv/kafka-python,gamechanger/kafka-python,xiaosl/kafka-python,KetsuN/kafka-python,coldeasy/kafka-python,barricadeio/kafka-python,zackdever/kafka-python,wikimedia/operations-debs-python-kafka,Yelp/kafka-python,Aloomaio/kafka-python,DataDog/kafka-python,barricadeio/kafka-python,mutability/kafka-python,trbs/kafka-python,mumrah/kafka-python,vshlapakov/kafka-python,ianawilson/kafka-python,reAsOn2010/kafka-python,oscarhealth/kafka-python,wikimedia/operations-debs-python-kafka,Aloomaio/kafka-python,ecanzonieri/kafka-python,alexzhang2015/kafka-python,dpkp/kafka-python,mutability/kafka-python,dyv/kafka-python,coldeasy/kafka-python,nmandavia/kafka-python,CrowdStrike/kafka-python,reAsOn2010/kafka-python,saaros/kafka-python,alexzhang2015/kafka-python,KetsuN/kafka-python,oscarhealth/kafka-python,vshlapakov/kafka-python,jessamynsmith/kafka-python,dpkp/kafka-python,scrapinghub/kafka-python,scrapinghub/kafka-python,grue/kafka-python