raft: proactively probe newly added followers

When the leader applied a new configuration that added voters, it would
not immediately probe these voters, delaying when they would be caught
up.

I noticed this while writing an interaction-driven test, which has now
been cleaned up and completed.

raft/raft.go

@@ -1527,10 +1527,18 @@ func (r *raft) switchToConfig(cfg tracker.Config, prs tracker.ProgressMap) pb.Co
 	if r.state != StateLeader || len(cs.Voters) == 0 {
 		return cs
 	}
+
 	if r.maybeCommit() {
-		// The quorum size may have been reduced (but not to zero), so see if
-		// any pending entries can be committed.
+		// If the configuration change means that more entries are committed now,
+		// broadcast/append to everyone in the updated config.
 		r.bcastAppend()
+	} else {
+		// Otherwise, still probe the newly added replicas; there's no reason to
+		// let them wait out a heartbeat interval (or the next incoming
+		// proposal).
+		r.prs.Visit(func(id uint64, pr *tracker.Progress) {
+			r.maybeSendAppend(id, false /* sendIfEmpty */)
+		})
 	}
 	// If the the leadTransferee was removed, abort the leadership transfer.
 	if _, tOK := r.prs.Progress[r.leadTransferee]; !tOK && r.leadTransferee != 0 {