This PR fixes a bug in static group membership. Previously we limit the `member.id` replacement in JoinGroup to only cases when the group is in Stable. This is error-prone and could potentially allow duplicate consumers reading from the same topic. For example, imagine a case where two unknown members join in the `PrepareRebalance` stage at the same time.
The PR fixes the following things:
1. Replace `member.id` at any time we see a known static member rejoins group with unknown member.id
2. Immediately fence any ongoing join/sync group callback to early terminate the duplicate member.
3. Clearly handle Dead/Empty cases as exceptional.
4. Return old leader id upon static member leader rejoin to avoid trivial member assignment being triggered.
Reviewers: Guozhang Wang <wangguoz@gmail.com>, Jason Gustafson <jason@confluent.io>