Make Ring.NewConsumer seq argument mandatory

There's no point in supporting a nil argument anymore.

downstream.go

@@ -661,15 +661,12 @@ func (dc *downstreamConn) welcome() error {
 	})
 
 	dc.forEachNetwork(func(net *network) {
-		var seqPtr *uint64
-		if sendHistory {
 		seq, ok := net.history[dc.clientName]
-			if ok {
-				seqPtr = &seq
-			}
+		if !sendHistory || !ok {
+			return
 		}
 
-		consumer := net.ring.NewConsumer(seqPtr)
+		consumer := net.ring.NewConsumer(seq)
 
 		// TODO: this means all history is lost when trying to send it while the
 		// upstream is disconnected. We need to store history differently so that

ring.go

@@ -31,27 +31,18 @@ func (r *Ring) Produce(msg *irc.Message) {
 	r.cur++
 }
 
+// Cur returns the current history sequence number.
 func (r *Ring) Cur() uint64 {
 	return r.cur
 }
 
 // NewConsumer creates a new ring buffer consumer.
 //
-// If seq is nil, the consumer will get messages starting from the last
-// producer message. If seq is non-nil, the consumer will get messages starting
-// from the specified history sequence number (see RingConsumer.Close).
-//
-// The consumer can only be used from a single goroutine.
-func (r *Ring) NewConsumer(seq *uint64) *RingConsumer {
-	consumer := &RingConsumer{ring: r}
-
-	if seq != nil {
-		consumer.cur = *seq
-	} else {
-		consumer.cur = r.cur
-	}
+// The consumer will get messages starting from the specified history sequence
+// number (see Ring.Cur).
+func (r *Ring) NewConsumer(seq uint64) *RingConsumer {
+	consumer := &RingConsumer{ring: r, cur: seq}
 	r.consumers = append(r.consumers, consumer)
-
 	return consumer
 }