Miscellaneous preparatory patches for group 0 discovery.
* scylla-dev/raft-group-0-part-2-v4:
raft: (service) servers map is gid -> server, not sid -> server
system_keyspace: raft.group_id and raft_snapshots.group_id are TIMEUUID
raft: (server) wait for configuration transition to complete
raft: (server) implement raft::server::get_configuration()
raft: (service) don't throw from schema state machine
raft: (service) permit some scylla.raft cells to be empty
raft: (service) properly handle failure to add a server
raft: implement is_transient_error()
6bdf8c4c46