Group coordinator state machine:

    - process received events
    - generate events to send
    - state
        - coordinator unknown
        - inactive
        - joined
        - 
        - rebalancing
        - active

coordinator unknown, GroupCoordinatorResponse -> coordinator unknown
coordinator unknown, GroupCoordinatorResponse -> inactive
inactive, JoinGroupResponse -> joined
, JoinGroupResponse -> syncing
syncing, SyncGroupRequest -> syncing
syncing, SyncGroupResponse -> active



KafkaClient router

    - send request
    - route request to node
      - all:
        - apiversions
        - saslhandshake
      - any:
        - metadata
        - groupcoordinator
        - offsetfetch / offsetcommit v0
        - createtopics / deletetopics
        - listgroups / describegroups
      - partition leader:
        - fetch
        - produce
        - offset
      - group coordinator:
        - joingroup
        - syncgroup
        - leavegroup
        - heartbeat
        - offsetfetch / offsetcommit v1+
    - encode request to bytes
    - queue bytes per node