A few inline comments for producer-ng functions

Also credit myself for reference implementations of the two channel idioms that
I ended up using. Having https://github.com/eapache/channels/ already written
and at hand made it a lot easier to reason about the design when I started
building this.

producer.go

@@ -239,6 +239,7 @@ func (p *Producer) Close() error {
 ///////////////////////////////////////////
 
 // singleton
+// dispatches messages by topic
 func (p *Producer) topicDispatcher() {
 	handlers := make(map[string]chan *MessageToSend)
 
@@ -287,6 +288,7 @@ func (p *Producer) topicDispatcher() {
 }
 
 // one per topic
+// partitions messages, then dispatches them by partition
 func (p *Producer) partitionDispatcher(topic string, input chan *MessageToSend) {
 	handlers := make(map[int32]chan *MessageToSend)
 	partitioner := p.config.Partitioner()
@@ -321,6 +323,8 @@ func (p *Producer) partitionDispatcher(topic string, input chan *MessageToSend)
 }
 
 // one per partition per topic
+// dispatches messages to the appropriate broker
+// also responsible for maintaining message order during retries
 func (p *Producer) leaderDispatcher(topic string, partition int32, input chan *MessageToSend) {
 	var leader *Broker
 	var output chan *MessageToSend
@@ -401,6 +405,8 @@ func (p *Producer) leaderDispatcher(topic string, partition int32, input chan *M
 }
 
 // one per broker
+// groups messages together into appropriately-sized batches for sending to the broker
+// based on https://godoc.org/github.com/eapache/channels#BatchingChannel
 func (p *Producer) messageAggregator(broker *Broker, input chan *MessageToSend) {
 	var ticker *time.Ticker
 	var timer <-chan time.Time
@@ -459,6 +465,7 @@ shutdown:
 }
 
 // one per broker
+// takes a batch at a time from the messageAggregator and sends to the broker
 func (p *Producer) flusher(broker *Broker, input chan []*MessageToSend) {
 	var closing error
 	currentRetries := make(map[string]map[int32]error)
@@ -560,6 +567,8 @@ func (p *Producer) flusher(broker *Broker, input chan []*MessageToSend) {
 }
 
 // singleton
+// effectively a "bridge" between the flushers and the topicDispatcher in order to avoid deadlock
+// based on https://godoc.org/github.com/eapache/channels#InfiniteChannel
 func (p *Producer) retryHandler() {
 	var buf []*MessageToSend
 	var msg *MessageToSend