Don't export newSyncProducerFromProducer

It has to modify the config out from under the real producer, so it isn't really
safe to use except in the specific circumstances of the other two SyncProducer
constructors.

sync_producer.go

@@ -16,7 +16,7 @@ func NewSyncProducer(addrs []string, config *Config) (*SyncProducer, error) {
 	if err != nil {
 		return nil, err
 	}
-	return NewSyncProducerFromProducer(p), nil
+	return newSyncProducerFromProducer(p), nil
 }
 
 // NewSyncProducerFromClient creates a new SyncProducer using the given client.
@@ -25,11 +25,10 @@ func NewSyncProducerFromClient(client *Client) (*SyncProducer, error) {
 	if err != nil {
 		return nil, err
 	}
-	return NewSyncProducerFromProducer(p), nil
+	return newSyncProducerFromProducer(p), nil
 }
 
-// NewSyncProducerFromProducer creates a new SyncProducer using the given producer as backing.
-func NewSyncProducerFromProducer(p *Producer) *SyncProducer {
+func newSyncProducerFromProducer(p *Producer) *SyncProducer {
 	p.conf.Producer.AckSuccesses = true
 	sp := &SyncProducer{producer: p}