Make it so that failures in `removenode`/`decommission` don't lead to reduced availability, and any leftovers in group 0 can be removed by `removenode`:
- In `removenode`, make the node a non-voter before removing it from the token ring. This removes the possibility of having a group 0 voting member which doesn't correspond to a token ring member. We can still be left with a non-voter, but that's doesn't reduce the availability of group 0.
- As above but for `decommission`.
- Make it possible to remove group 0 members that don't correspond to token ring members from group 0 using `removenode`.
- Add an API to query the current group 0 configuration.
Fixes#11723.
Closes#12502
* github.com:scylladb/scylladb:
test: test_topology: test for removing garbage group 0 members
test/pylib: move some utility functions to util.py
db: system_keyspace: add a virtual table with raft configuration
db: system_keyspace: improve system.raft_snapshot_config schema
service: storage_service: better error handling in `decommission`
service: storage_service: fix indentation in removenode
service: storage_service: make `removenode` work for group 0 members which are not token ring members
service/raft: raft_group0: perform read_barrier in wait_for_raft
service: storage_service: make leaving node a non-voter before removing it from group 0 in decommission/removenode
test: test_raft_upgrade: remove test_raft_upgrade_with_node_remove
service/raft: raft_group0: link to Raft docs where appropriate
service/raft: raft_group0: more logging
service/raft: raft_group0: separate function for checking and waiting for Raft
563998b69a