kvserver: designate follower to campaign on Raft leader removal

[erikgrinaker]

Currently, when a leader removes itself from the range, we let the first voter in the range descriptor campaign to elect a new leader without waiting out the election timeout. However, this is problematic for a couple of reasons: the first replica may lag or be unavailable (in which case we have to wait out the election timeout), and when we enable PreVote+CheckQuorum (#92088) the current PreVote campaign will likely fail since the other followers have heard from the leader recently.

We can address both of these issues by choosing an up-to-date designated follower when we propose the conf chance, including it in ConfChange.Context, and have that follower do a TimeoutNow campaign (bypassing prevote) once it applies the conf change.

cockroach/pkg/kv/kvserver/replica_raft.go

Lines 2596 to 2602 in 87d6547

	// If the leader is no longer in the descriptor but we are the first voter,
	// campaign.
	_, leaderStillThere := desc.GetReplicaDescriptorByID(roachpb.ReplicaID(st.Lead))
	if !leaderStillThere && storeID == desc.Replicas().VoterDescriptors()[0].StoreID {
	log.VEventf(ctx, 3, "leader got removed by conf change")
	return true
	}

Jira issue: CRDB-28762

Epic CRDB-25199

[blathers-crl]

cc @cockroachdb/replication