Initialize `next_idx` in Raft::new

[Fullstop000]

In Raft::new, we always create a ProgressSet with setting next_idx 1 in each Progress: https://github.com/tikv/raft-rs/blob/f7f7d0121132ef6406b59fc774778285cd1477d6/src/raft.rs#L251-L255

In the situation where recovering the raft node, I think maybe we can just use the current Snapshot's meta index as same as the implementation in restore_raft : https://github.com/tikv/raft-rs/blob/f7f7d0121132ef6406b59fc774778285cd1477d6/src/raft.rs#L1984-L1987

[hicqu]

When a peer becomes leader, it will reset next_idx to last_index + 1 for all followers. So I think it's OK?

[Fullstop000]

I just want to make the implementation stay consistent with resotre_snapmeta :). Feel free to close this.