fix: ensure heartbeats only stop after cleanup
fix: ensure heartbeats only stop after cleanup

Sarama would immediately stop heartbeating on rebalance. If the session
teardown took longer than the session timeout, it would get evicted from
the group.

This can be fixed by ensuring that heartbeats only stop _after_ the
session has completed its cleanup process.

Contributes-to: #1608

Shopify/sarama,DataDog/sarama,Shopify/sarama,DataDog/sarama