This patch adds support for recreating group 0 after losing
majority. This is the only part of the new Raft-based recovery
procedure that touches Scylla core.
The following steps are necessary to recreate group 0:
1. Determine the new group 0 members. These are alive nodes that
are normal or rebuilding.
2. Choose the recovery leader - the node which will become the
new group 0 leader. This must be one of the nodes with the
latest persistent group 0 state.
3. Remove `raft_group_id` from `system.scylla_local` and truncate
`system.discovery` on each live node.
4. Set the new scylla.yaml parameter - `recovery_leader` - to Host
ID of the recovery leader on each live node.
5. Rolling restart all live nodes, but the recovery leader must be
restarted first.
In the implementation, restarts in step 5 are very similar to normal
restarts with the Raft-based topology enabled. The only differences
are:
1. Steps 3-4 make the restarting node discover the new group 0
in `join_cluster`.
2. The group 0 server is started in `join_group0`, not
`setup_group0_if_exists`.
3. The restarting node joins the new group 0 in `join_topology` using
`legacy_handshaker`. There is no reason to contact the topology
coordinator since the node has already joined the topology.
Unfortunately, this patch creates another execution path for the
starting logic. `join_cluster` becomes even messier. However, there
is nothing we can do about it. Joining group 0 without joining
topology is something completely new. Having a few small changes
without touching other execution paths is the best we can do.
We will start removing the old stuff soon, after making the
Raft-based topology mandatory, and the situation will improve.
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