Not wait until updateAssignmentMetadataIfNeeded returns true, but only call it once with 0 timeout. Also do not return empty if in rebalance.
Trim the pre-fetched records after long polling since assignment may have been changed.
Also need to update SubscriptionState to retain the state in assignFromSubscribed if it already exists (similar to assignFromUser), so that we do not need the transition of INITIALIZING to FETCHING.
Unit test: this actually took me the most time :)
Reviewers: John Roesler <john@confluent.io>, Bill Bejeck <bill@confluent.io>, Bruno Cadonna <bruno@confluent.io>, Sophie Blee-Goldman <sophie@confluent.io>, Jason Gustafson <jason@confluent.io>, Richard Yu <yohan.richard.yu@gmail.com>, dengziming <dengziming1993@gmail.com>