A series of refactors to the `raft_group0` service.
Read the commits in topological order for best experience.
This PR is more or less equivalent to the second-to-last commit of PR https://github.com/scylladb/scylla/pull/10835, I split it so we could have an easier time reviewing and pushing it through.
Closes#11024
* github.com:scylladb/scylla:
service: storage_service: additional assertions and comments
service/raft: raft_group0: additional logging, assertions, comments
service/raft: raft_group0: pass seed list and `as_voter` flag to `join_group0`
service/raft: raft_group0: rewrite `remove_from_group0`
service/raft: raft_group0: rewrite `leave_group0`
service/raft: raft_group0: split `leave_group0` from `remove_from_group0`
service/raft: raft_group0: introduce `setup_group0`
service/raft: raft_group0: introduce `load_my_addr`
service/raft: raft_group0: make some calls abortable
service/raft: raft_group0: remove some temporary variables
service/raft: raft_group0: refactor `do_discover_group0`.
service/raft: raft_group0: rename `create_server_for_group` to `create_server_for_group0`
service/raft: raft_group0: extract `start_server_for_group0` function
service/raft: raft_group0: create a private section
service/raft: discovery: `seeds` may contain `self`
76d20aeb96