In preparation for supporting IP address changes of Raft Group 0:
1) Always use start_server_for_group0() to start a server for group 0.
This will provide a single extension point when it's necessary to
prompt raft_address_map with gossip data.
2) Don't use raft::server_address in discovery, since going forward
discovery won't store raft::server_address. On the same token stop
using discovery::peer_set anywhere outside discovery (for persistence),
use a peer_list instead, which is easier to marshal.
Closes#11676
* github.com:scylladb/scylladb:
raft: (discovery) do not use raft::server_address to carry IP data
raft: (group0) API refactoring to avoid raft::server_address
raft: rename group0_upgrade.hh to group0_fwd.hh
raft: (group0) move the code around
raft: (discovery) persist a list of discovered peers, not a set
raft: (group0) always start group0 using start_server_for_group0()
08e654abf5